Before considering the Air - I bought one but had to swap out - you will HEAVILY need to consider how much data backups are on DVD/CD that you'll need to migrate over to a stable HDD array (more than 1 just in case) - or even to the cloud. You'll also need to consider how much data you're comfortable with on Box.net, DropBox, and other services (Me.com) to which you can readily retrieve data. For documents, pictures, music, and short movies (non DVD length) this works out for just about everybody when at home or in the city. However if you require larger files to which to edit and use heavily, possibly even creating large files, you'll need to consider:
1- a data stick or your mobile smartphone as a wlan router for tethering (battery life)
2 - how large are the files you'll be doing this with over the lifespan of your mac
(PS: get a time capsule or similar setup to talk back to your OWN array of online accessible storage).
The bonus is the 1440x900 resolution display on the MBA 13" antiglare vs the 1280x 760 on the 13" MBP - a GLARING oversight by Apple which MUST be fixed for the 13" bambino MBP.
I never thought AntiGlare would matter but since using it for 4mths on the 13" MBA Ultimate that I purchased and eventually traded for a 2010 MBP 15" with Hi res display ... I'm NEVER going back!