Well, well! I know its just after 3 in the morning when this post was made, but my upload is back to normal (for how long I don't know). Just uploaded a 5mb file, twice, to my briefcase and no problem, apart from a few pauses but the upload completed!
Also tried a 3 file multiple upload (1mb, 2.5mb and 1.2mb) via Yahoo briefcase and those too uploaded
Damn! If they've fixed the problem, who we gonna bitch at now? And does it only work early in the morning?