So, it sounds like you can download a song and "store" it on your iPhone or iPod touch. Does anyone think that this feature of "storing" a downloaded file on your iPhone will lead to the ability to download and "store" lets say attachments in mail?
Well you can already store attachments by just not deleting the e-mail. Right now, what you're looking for would be the snowball-fight-in-hell iPhone firmware update.
Seriously, if they do add a file system for other types of data files to iPhone, it won't come directly out of the WiFi Music Store. It will be a later feature added if Apple thinks demand warrants it. A standard file system for spreadsheets or what have you would complicate the iPhone and I think they're very, very touchy -- pardon the pun -- about complicating the use and interface of iPhone.