I'm personally planning to hold out until Lion comes out (I've used my G5 iMac for this long, I can probably make it a few more months
) but my main reason to wait is my delusional hope that Apple will offer a higher-res, matte display option for the MPB-13, especially if
enough of us send in feedback politely requesting it. That, or there's a slim chance we'll see updated MBAs by then (pure speculation). Either way I'll get a fresh Lion installation and not have to bother with updating SL. Yeah, I know 10.7.0 is likely to have some bugs, but...