This kde bug: http://bugs.kde.org/140870 is present in all of gentoo's umbrello ebuilds. I used the patch in that bug and it fixed it. I'll attach the patch.
Created attachment 113635 [details, diff] umbrello.diff
Thanks for bringing this to our attention and for supplying the patch, David. I've just committed kde-base/umbrello-3.5.5-r1 and kde-base/umbrello-3.5.6-r1 with this patch.
BTW, there are still problems with this version of umbrello. Supposedly they are fixed in later versions. I'm not really clear of the policy on the ~x86/x86 flags and whether or not just being able to compile and start up an application qualifies it as stable. But if something more is required to be stable, I definitely wouldn't give it to this version of umbrello because it is still buggy. I don't use it day-to-day so I really can't tell you what they were.