If you sync your emails and contacts via an exchange server, you could email the cab file to yourself so it would always be there. You might have to do this once every 30 days depending on how many past emails you tell it to sync. You might could also attach the file to a calendar item (far into the future) or contact that gets synced.

Just one idea. Haven't tried it, but I email cabs to myself routinely. If you don't sync via exchange, maybe it would work with wireless sync or something similar.

EDIT: Even more obvious, that just hit me. You could come to pdaphonehome on your i760 and download it from the appropriate thread. Granted, navigating and searching on the phone is not as easy as on the desktop, but you'll get there.