I have tried using games-server/monopd-0.9.3-r1 on amd64 and found no problems whatsoever. The ebuild specifies KEYWORDS="~ppc x86". Please, add amd64 or ~amd64 to the keywords. Thank you.
This game still compiles and works fine on my amd64 machine. Requesting amd64 keyword!
+ 15 Dec 2011; Agostino Sarubbo <ago@gentoo.org> monopd-0.9.3-r1.ebuild: + Add ~amd64, wrt bug #249476 +