I've got a 13" 2009 MBP (8GB, 2.53Ghz). A lot of the time it runs a Win 7 web dev environment (Visual Studio, SQL Server databases, various editors, Photoshop etc.) under Parallels Desktop. It performs at least as well as a high-end Wintel laptop would if not better.
On the Mac end of things, I also mess around with Garageband and Coda, edit video and photos with iMovie and am learning Objective-C using XCode. And also for general use (web, email, officey-stuff, iTunes, DVD viewing etc). The laptop handles all this extremely well and although its a couple of years old now, I see no reason to upgrade to a newer model.
However, I'm very taken with the new (Summer '11) Macbook Air - they're light as a feather and most appealing. On paper, the 1.8Ghz i5 looks like its a bit weedy, but it has SSD. Is it going to perform as well or better than my current machine for the sorts of things I do with it or should I just invest in an SSD for my MBP?
UPDATE: Following the advice from @Benjamin Dubois and @user9902 to install an SSD in my current machine I did just that. For anyone who is wavering between buying an Air (and the 2012 editions are considerably more appealing than the 2011 model that seduced me at the time of posting this question) I would strongly suggest trying this option before splashing out big money on shiny new toys. After the installation of a sub-£100 128GB SSD the MBP absolutely flies. I would recommend this option without reservation.