In terminal enter this:
and repeat for each logical core (use command+T for a new tab). i5 MBPs have 4 logical cores.
Also, turn up LCD and keyboard brightness, use SMCFanControl to set the fans to max (if they are not already), use a bluetooth mouse, transfer large files over wifi, play music at full volume, burn a DVD, play Portal, and charge 2 iPads (or 3 if you have a 17"). If you have an iPod that will charge by Firewire, charge that too (or two of them, if you happen to have two FW ports). Doing all this the battery should last about 5 minutes.
edit:
Actually, it would be interesting to see if a MBP could really handle all of the above. Maybe I'll try it some time using resistors in the USB ports since I don't have two iPads.
iPads: 10W each
iPods: 2.5-5W each (assuming 1st gen iPod Touch on FW)
CD drive: 5W max
Hard Drive: 3W
CPU: 35W (that's the TDP for mine at least)
GPU: 20W?
Wifi: 1W?
Speakers: 1-2W?
Display: 10W?
Expresscard: ~4W
Total: 110W (worst-case)
The MBPs come with an 85W Magsafe, which means 85W is the max load they expect under normal conditions. That implies that the MBP would start draining the battery even if plugged in in this configuration, assuming the computer's power regulation can handle the load at all. I have AppleCare, so I might as well try it!
Edit2: I almost forgot to add the Expresscard slot power.