I had to jumpstart a Prius the other day. It was kind of tricky, and because the owner helped me do it, I'm still not sure how we got it to work. Tom had some problems yesterday with another one and that call turned into a GOA. I did a little reading on the Web about this topic and it looks like different hybrids are handled differently.
By suggestion to you if you ever run into a situation like this is to read the owners manual. There should always be information in there about a "Jump Starting Procedure".
Here are some instructions specific to the Prius I found online:
Should you need it, Prius can be jump-started with any standard 12-volt DC power source, and it actually requires less power than a conventional car. Simply connect the cable clamps to access points under the hood (which are connected to the auxiliary battery near the cargo area) and energize the computer. Then press the Start button to turn the car on. Please see your Owner's Manual for further instructions.