SVN - unable to merge branch back into trunk - numerous tree-conflicts

Sounds like you're using the pre-1.5 merge style and trying to reintegrate the branch into trunk. In that case, what you want to do is first ensure all the trunk changes have been merged in to the branch, and then instead of range-merging the branch to a working copy that points to the trunk, you want to merge "FROM trunk@HEAD TO branch@HEAD" with the working copy pointing to trunk. In essence:

"Give me all the changes I'd need to make trunk identical to branch".

This works if you've already merged all the trunk changes to the branch, because then the only difference between trunk and branch are the changes made in the branch.

Make sense? :)

I investigated the same problem. It is "feature" in Tortoise SVN 1.6.5. TortoiseSVN 1.5 works fine with our repositoty (SVN 1.5). TortoiseSVN 1.6.5 when rebasing adds files from mainline as NEW (without saving merge-history).
And reintegrating branch resuls in treating those files as conflicting with mainline.

I solved the problem by using feature of TortoiseSVN 1.6 "reintegrate branch". It's specifically purposed for feature-branches.

-- Alexey Korsun