According to http://developer.gimp.org/ChangeLog, CVS GIMP requires >=gtk+-2.8.17 and >=pango-1.12.2 as of 2006-07-07. Ebuild states only gtk+-2.8.8 and pango-1.4, so the emerge fails during configure.
Err, using live CVS ebuild really assumed you won't run stable dependencies. :P
(In reply to comment #1) > Err, using live CVS ebuild really assumed you won't run stable dependencies. :P Nonsense. This is, of course, a bug and will be fixed once I can find the time.
Fixed in CVS. Thank you for reporting this.