If I recall correctly (from the Tortoise SVN's commit log I saw), Tortoise SVN added this feature in January where you can edit a file and have TSVN restore it after commit (aka Commit only parts of a file).
TSVN's documentation also lists this feature now: http://tortoisesvn.net/docs/nightly/TortoiseSVN_en/tsvn-dug-commit.html
I am running latest version of TSVN (1.7.6) on a Win 7 64 bit machine, but I cannot see this option anywhere while committing a file. Am I missing something, or has this feature not been shipped??