For what it's worth, I now have a 32gig SD card planted in my SD card slot and it's working just fine.
Would highly recommend to anyone with a w510 to get a decent size SD card and have it permantly sitting in the slot (keeps crud from getting in there if you're a road warrior). At a mere $90 for an extra 32gigs, you can't go wrong.
Am now looking at maybe getting one of the ExpressCard storage devices, but find the price (~$150 for 32GB) a bit unpalatable.
Actually I just received an update for Win7 64bit (KB976422). This update indicates that it supports SD cards that are larger than 32GB. Haven't tried it yet... don't currently have a SD card larger than 32GB.