I had same problem.

One night, went to do my banking... opened the lid, 5 seconds then off...

My Macbook Pro had been known to do funny things on waking, so I tried again, 5 seconds then off...

Restarted, power button, battery out and in, ram... I tried everything.

5 seconds then off.

Took it into Apple, they replaced the logicboard at $1400... thank god for Applecare.

Got it home, was using for 10 minutes then off.

Took it in again, they replaced the logicboard at $1400 and the topcase... once again, thank god for Applecare.

Got it home... Worked mostly fine, occasionally it wouldn't wake properly from sleep... but a week later the DVD drive wouldn't allow disks to be inserted.

Took it back to Apple, they replaced the DVD drive... once again, thank god for Applecare.

Got it home, and with only 15 days remaining on my warranty ran into some more wake issues and graphics card.

I rang up, complained, spoke to Apple "Customer Relations", and now im getting it "Replaced", my three year old Macbook Pro replaced by a new unibody.

THANK GOD FOR APPLECARE.

So take it in, get it fixed, and if you have a string of problems like me, persist (nicely) for a replacement!