How does iPad choose between wifi and 3G when both are 'on'?

Easy peasy question.

I have 02 and Wifi going on my iPad. I know I can use 02 out and about and I know I can use wifi at home etc.

But when I'm at home how do I make sure I'm using my wifi for data and not my cellular connection???

I've been turning my cellular to 'off' when at home, just in case, but I don't want to have the faff of turning it off and on, so it would be nice to know how the iPad chooses between wifi and 3G when both data access points are active.

Also I've had no wifi troubles at home at all. so far... The acid test will be setting it up at work as they have proxyies for everything, but obviously I don't have proxies at home. I'm not sure if the settings screen can cope with different proxies dependent upon different wifi networks... * grin *