i don't understand what's wrong with a merge commit? it would point to two branches (in git now, but i think also in svn with the merge tracking in 1.5)
I think it's best to keep the "official" history strictly linear; branches and merges are difficult to follow in logs, and especially in annotations. It's natural that a single line is easier to visualize and follow than a tree.
Combining that with the goal of keeping history as detailed as possible, I want my complete development branch to be folded onto the main trunk as if all the commits happened in a single instant (except git can keep the real dates).
This means I'd have to rebase and resolve conflicts for all my development commits, but that is also work that makes the main trunk easier to follow in the future. (In practice I'd continuously rebase my development branch to keep it on top of the main trunk.)