my colleagues found a issue with the definition of external diff programs today which I can reproduce. So it still exists in TortoiseSVN 1.7.7.
The issue is, that the external diff program defined (WinMerge in our case) is being used as expected for comparing two revisions past, but for comparing the working copy with a past revision Tortoise's diff program is being used. This happens from the graph as well as from the log dialogue.
The graph has an additional issue: depending on whether you've selected the working copy first and the other revision afterwards or vice versa the context menu has the view diff option or not. In my eyes that's a bug as well.
Has anybody hints for these two issues? Are they perhaps a bug? At least the behaviour is unexpected.