I wouldn't dream of it, personally.
if you're just using a mbp for web browsing then I'd say sell it and buy a standard macbook and spend the rest on coke and hookers.
i have had my macbook pro now for a few months and really like but just been to have a look at the air and i think i have fallen in love with it should i sale my pro and get a air
only really use my mac for web etc no games or any thing
Maybe some more RAM for the MB and less expensive hookers?
Unless you really really don't need the extra power of the MBP then I think you'd be mad to trade. If you're that desperate for a small laptop then save up a couple of hundred quid and buy an eeePC and keep the MBP.
Well I had a MB and a MBP and am typing on a MBA at the moment. For what I do, the MBA is brilliant. And it isn't "slow" as everyone else says. It is at least as fast the MB in terms of performance, even if not on paper. And, it hasn't once been embarrassed by the pro's performance.