How often does the iPhone look for wifi?

[jschwa]

I have some questions about how often an iPhone looks for a wifi signal. I appreciate any insight you all can provide.

When an iphone is in sleep mode:
Does it ever look for a wifi signal?
Does it ever connect to a wifi signal if it finds one. Ex. It wants to download mail, and looks for wifi before using the cell network.
If it is connected to a wifi network when it goes into sleep mode, how long does it stay connected to it?

When an iPhone is awake:
How often does it look for a wifi signal?
Does it only look for wifi when it needs to transfer data or is it periodically sniffing for wifi to see if it finds a known network to connect to?

Thank you!