Marc Orchant trusted Google, at least until he heard about a small number of folks who seem to have lost their Gmail accounts, now he’s madly using Google’s POP3 access to grab a copy of all the email he has stored in Gmail.
I use Gmail, I also use Outlook to access Gmail and make sure I get a copy of everything that is store in Gmail, keeping anything that would hurt to lose safely stored on my laptop as well as Gmail. Everything I have on this site exists in two places, at least. Every photo I have on Flickr, I have stored on a PC and backed up to an external drive as well.
I might trust most of these services to do what I expect them to do, but none of them are perfect. It never hurts to have a backup plan.