Dropped mine off at my local Nokia care centre (Cellular Services on Reigate Road for anyone who's near my neck of the woods) this afternoon. No problem at all, they seemed well aware that a number of N900's have had USB problems.

They have said that they will check the port and repair it on-site if they can, if not they will send it off to Nokia for either repair or replacement as appropriate.

I'll keep you posted on progress.