I have installed qt-2.3.1 and kde-2.2.2-r1. When I try to install kdevelop-2.0.2, emerge wants to install qt-docs-3.0.1 instead of 2.3.1.
It's possible for qt 2 & 3 to survive together and is actually the way it will have to be with some programs using one and some using another. The problem I saw with the ebuild is that when listing what packages will be installed it has an "upgrade" flag on qt 3 as opposed to a "new" flag. I didn't want to chance trying it out to see if it would leave qt 2 but if it doesn't the system would be in some trouble.
There's an "upgrade" flag because portage doesn't yet have really good slot support. It will soon though. qt 2 and 3, and qt-docs 2 and 3, live together peacefully and there are no conflicts or problems. I'll fix the kdevelop depend now (change it from >=qt-docs-2.3 to =qt-docs-2.3*). This way the qt-docs installed will always belong to qt2.x. Thanks for reporting.