Japanese blog Mac Otakara claims sources have told them Apple is testing a MacBook Air with an A5 processor, the same CPU powering the iPad 2. While the machine performed "better than expected" according to their source, Mac Otakara says it's unclear whether this test machine was running Mac OS X or iOS. At any rate, the site claims this A5 variant MacBook Air has been created "just for experiment." Mac Otakara has been a reliable source of rumors in the past, having been one of the first sites to accurately depict the updated form factor of the iPad 2.
If Apple has indeed built such a device, it's unlikely the company intends to put it on the market. While the A5 processor is powerful enough under iOS, in terms of raw performance it pales in comparison to even the least powerful Intel chips. For example, the iPad 2's Geekbench score is around 720, while the lowest-rated MacBook Air processor scored over 2000. Benchmarks don't tell the whole story, of course, but they're a fairly reliable predictor of the general performance you can expect to get from a machine.
If Apple's experimenting with ARM processors in its laptop line, it's very unlikely that it's a simple matter of trying to run Mac OS X on the A5 and more likely that Apple is looking for ways to run iOS in a notebook form factor. This could be something as simple as a device that looks like a MacBook Air but has a touchscreen and runs iOS -- basically an iPad with an attached keyboard. Or Apple could have more ambitious plans in mind, with an A5 processor running as a sort of sidecar to a standard Intel CPU, with Mac OS X and iOS both capable of running on the same machine on their respective CPUs. Under such a scheme, iOS might run as a sort of "layer" atop Mac OS X -- rather like Dashboard does now, but more like Dashboard after a serious dose of growth serum.
From what we've seen of Mac OS X Lion thus far, it already looks as though Apple's looking at ways of merging OS X and iOS as far as the software's concerned. If Mac Otakara's sources are correct, it seems it's just a matter of time before the hardware follows.