Android (Froyo) has had built-in wireless 'hotspot' tethering for a year now. Works like a charm.
Users of wireless 'hotspot' tethering over their smartphones ought to be aware, though, that although you can connect your laptop or desktop over WiFi via your iPhone 4 to the internet now, the connection from the phone to the internet is over 3G/GPRS/CDMA. In other words: it will eat into your data plan!
Now don't turn around and tell me you don't care because you have an 'unlimited data plan', because 'unlimited data plans' are not unlimited at all! They all apply a socalled 'fair use policy'. Which means there IS a cap! And you will be notified when you exceed it, and you will be cut off (or charged, dearly, for the excess) if your data use persists at that level!
I got a 'stern' warning after only 10 hours (in December) of streaming videos over wireless 'hotspot' tethering!
So I don't stream video over a 'portable wireless WiFi hotspot' (my phone) to my MBP anymore!