The final emerge output looks like this:

** (../../class/lib/net_2_0_bootstrap/mcs.exe:14146): WARNING **: Missing method .ctor in assembly /usr/lib/mono/gac/I18N/2.0.0.0__0738eb9f132ed756/I18N.dll, type System.Runtime.CompilerServices.RuntimeCompatibilityAttribute

** ERROR **: Can't find custom attr constructor image: /usr/lib/mono/gac/I18N/2.0.0.0__0738eb9f132ed756/I18N.dll mtoken: 0x0a000048
aborting...
Stacktrace:

  at (wrapper managed-to-native) System.AppDomain.LoadAssembly (string,System.Security.Policy.Evidence,bool) <0x00004>
  at (wrapper managed-to-native) System.AppDomain.LoadAssembly (string,System.Security.Policy.Evidence,bool) <0xffffffff>
  at System.AppDomain.Load (string) <0x00018>
  at (wrapper remoting-invoke-with-check) System.AppDomain.Load (string) <0xffffffff>
  at System.Reflection.Assembly.Load (string) <0x0001b>
  at System.Text.Encoding.InvokeI18N (string,object[]) <0x0007c>
  at System.Text.Encoding.GetEncoding (string) <0x0015d>
  at System.Text.Encoding.get_Default () <0x0006b>
  at System.Console..cctor () <0x000e0>
  at (wrapper runtime-invoke) System.Object.runtime_invoke_void (object,intptr,intptr,intptr) <0xffffffff>
  at Mono.CSharp.Report..cctor () <0xffffffff>
  at Mono.CSharp.Report..cctor () <0x0000a>
  at (wrapper runtime-invoke) System.Object.runtime_invoke_void (object,intptr,intptr,intptr) <0xffffffff>

Native stacktrace:

        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono [0x815f1ab]
        [0x4001c440]
        /lib/libc.so.6(abort+0x108) [0x40148dc8]
        /usr/lib/libglib-2.0.so.0(g_logv+0x43f) [0x4007c0ef]
        /usr/lib/libglib-2.0.so.0(g_log+0x29) [0x4007c129]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono(mono_custom_attrs_from_index+0x1eb) [0x80c14cb]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono(mono_custom_attrs_from_assembly+0x44) [0x80c17a4]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono(mono_assembly_load_from_full+0x427) [0x809b597]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono(mono_assembly_open_full+0x2ce) [0x809bafe]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono(mono_assembly_load_full+0x98a) [0x809cb7a]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono [0x80b90a2]
        [0x404e500a]
        [0x404e4f89]
        [0x404e4f3e]
        [0x404e4e64]
        [0x404e4acd]
        [0x404e3de6]
        [0x404e3a3c]
        [0x404e30c1]
        [0x404e1236]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono(mono_runtime_class_init+0x198) [0x80dacb8]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono [0x814b5af]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono [0x8077e5a]
        [0x402ca032]
        [0x404e09be]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono(mono_runtime_class_init+0x198) [0x80dacb8]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono [0x813f2d2]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono [0x81499db]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono [0x814b34e]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono [0x814b94b]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono(mono_runtime_exec_main+0x9c) [0x80dc21c]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono(mono_runtime_run_main+0x1b3) [0x80dc4c3]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono(mono_main+0xe30) [0x8058da0]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono [0x8057a82]
        /lib/libc.so.6(__libc_start_main+0xd8) [0x40134838]
        /var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mono/mini/mono [0x80579d1]

=================================================================
Got a SIGABRT while executing native code. This usually indicates
a fatal error in the mono runtime or one of the native libraries 
used by your application.
=================================================================

make[8]: *** [../../class/lib/net_2_0_bootstrap/Mono.Security.dll] Aborted
make[8]: Leaving directory `/var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mcs/class/Mono.Security'
make[7]: *** [do-all] Error 2
make[7]: Leaving directory `/var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mcs/class/Mono.Security'
make[6]: *** [all-recursive] Error 1
make[6]: Leaving directory `/var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mcs/class'
make[5]: *** [all-recursive] Error 1
make[5]: Leaving directory `/var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mcs'
make[4]: *** [profile-do--net_2_0_bootstrap--all] Error 2
make[4]: Leaving directory `/var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mcs'
make[3]: *** [profiles-do--all] Error 2
make[3]: Leaving directory `/var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/mcs'
make[2]: *** [all-local] Error 2
make[2]: Leaving directory `/var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1/runtime'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/var/tmp/portage/dev-lang/mono-1.2.2.1/work/mono-1.2.2.1'
make: *** [all] Error 2

!!! ERROR: dev-lang/mono-1.2.2.1 failed.
Call stack:
  ebuild.sh, line 1576:   Called dyn_compile
  ebuild.sh, line 945:   Called src_compile
  mono-1.2.2.1.ebuild, line 89:   Called die

!!! compile failed
!!! If you need support, post the topmost build error, and the call stack if relevant.