I've got the same problem - Magic mouse, OS 10.7.5, and no scrolling.
Our tech support said it may be corrupted mouse driver preferences.
His 1st suggestion was to delete the mouse from bluetooth, and then re-connect it. That usually re (boot? reinstall?) the driver. Tried it, with no luck.
When he's back in the office, I'll ask him to follow up. When I find out the problem I'll post up here with solution.