Check out Mail in Leopard. Now if you don't start giggling, and I mean a good kinda giggle, the "holy crap that is REALLY cool" giggles, go read "Getting Things Done" and watch the video again. Giggling now, aren't you?
Notes, To-Do's in an email app might seem silly. True, I'd settle for a rock-stable database. But maybe between Time Machine and general improvements to CoreData and the OS will help... Notes and To-Do's in Mail does one thing nice as well: integrates with iCal. This is super-nice because you can share calendars, online or with your iPod... With something like Life2Go you can add Mail emails and docs to your iPod. This is getting quite nice, and darn handy.
When you add the inevitable QuickSilver action to add a note or to-do, you will really have a GTD system in OS X. I would still love to see tags in email, and I hope notes can be added directly TO an email. That is so handy...
one man's journey into creating gibblybits