Same thing happening to me too. MBP late 2008. Unresponsive keyboard and trackpad for 10-15 secs or so. Have never had this problem before 10.6.3. I have tried resetting PRAM and that didn't help. I just reset my SMC, so we'll see if that holds.
I was working most of the day just fine with an external keyboard and mouse. Then as soon as I started using it at home, consistent lockups. Amazingly, it hasn't done it all since I started typing this, so maybe it was the SMC.
Nope, just happened as I typed that last line. If anyone has any other suggestions, please reply.