Someone tested a beagleboard of same rough time as N900, and it wasn't stable beyond 800MHz.

What makes things confusing, is that TI is constantly making new models/revisions without changing the model number. Thus a CPU coming off TI's production line today might very well run at over 1ghz for some time.

The original beagleboard came clocked at 500MHz, and the next batch sold (if it isn't being sold already) will be 720MHz. With the same model cpu, just a different revision of it.