[kgutteridge]

I have modded 15" pro

1) I have a 128gb SSD for the OS etc where the optical drive was and a 750gb SATA drive replacing the stock one

2) 8GB ram if you use parallels or eclipse etc this a no brainer for the price/ease

3)the hires matt screen (1680x1050) means you can use outside

So whilst not as portable as the airs, it never lacks power and is user upgradeable, my slight fear of the air is you would always be waiting on Apple for an upgrade and it might not quite fit your requirements (large disk or ram)

I also use a backpack anyway so really apart from when I ram it full,to avoid having to check a bag on a plane, the air wouldn't really make that much difference personally, that said its a tough choice especially if you don't need the ram