This site lets you document expenses while mobile (or not mobile if you so desire). You can easily have different "reports" of "expenses", so what I do is have a "Check Register" report (read category), and then when I write a check, I can just email "exp WidgetOnSale 24.95 for the Blackberry project", or twitter it, or sms it, or IM it, or even phone it in via Jott (or Sandy)!
Once the expense is sent, I can manage them from their web interface, and "move" it from my "Inbox" category (report) to my "Check Register". You get the idea. Pretty handy.
very interesting... Looks like it could be useful. Would be great if the mileage log comes through. That's a component I could really make good use of.