Join Date: Mar 2010
Location: Miami, FL
Bikes: 08 Giant OCR C2, 05 Specalized Hardrock Sport Disk
Check out Garmin's forerunner series. They are worn like a watch, are GPS enabled, and most units can be used with a HR monitor and a speed/cadence sensor for your bike. They are also waterproof. So you can put on a Forerunner, and it will work as a running computer and then when you get to the bike it will pick up the data from the cadence sensor and become a bike computer - and when it is time to swim no worries, it is waterproof to IPX7 standards.
Look at the Forerunner 305 and Forerunner 310XT - one of those two is probably what you want. The 405 and 405CX are not really multi-sport models, and the 205 can not use bike specific stuff like a cadence sensor.
Use the "compare models" to find the one with the features you think you need, then download the user manual to make sure it will work for you before you buy.