No Reason to Switch
Although I much prefer to Aperture to Lightroom (see my long thread with Trevor the Canadian), there is no reason to switch. Both are good programs and each beats the other at various things. (And that will change with various releases.)
The only reason to switch would be if there is some feature you really can't live without, or the Lightroom's modular workflow starts to bug you.
In my case, I lust after Lightroom's new noise reduction but would never put up with the user interface and I could not live without soft proofing for printing.