Funny... I had this issue with my Samsung i760 before I got my hands on a Samsung Saga. I originally thought it was a programming issue with WM6 on the i760 but now that I have the Saga, the same thing is happening.

I'm pretty sure it has something to do with SPB mobile shell, it's the only thing that both phones had in common.

That frenetic vibrating drives me nuts, and I wish I could figure out exactly why that is happening. I will try to change the standard alarms/rings and see what that does.