You know...if 16G is not enough, they do make a 32G model?
Just saying.
This is the way I look at it. You have a 32G iPhone, MobileMe or Box.net at 20G, and Dropbox app on your iPhone and computer, then you have access to at least 52G of memory with your iPhone. IMHO if you need more than 52+ GB of memory on your device.....you need a laptop!
I had Treos for years and you know how many SD cards I carried with me? One. the one that was in my Treo. I kept losing...errr.... misplacing all the other ones. I would buy the largest SD card I could and that was it!
SD card slot...Mehhh.