AnonymousUser is correct.. some updates it FORCES over wifi, since they can be large.
As he said.. you will want to go into SETTINGS > ABOUT > SOFTWARE UPDATE.
This will force it to go out and check for an update.
Now.. as for WHAT updates it has. With almost all ANDROID devices its at the CARRIERS discgression for releases for them. When say... SONY releases an update for the device.. it goes out to the carrier. They carrier checks it, makes sure it works on their network, etc... THEN they MAY release it for download for their users.
(this is pretty mucht he way for all android devices in canada and the US, unless you mod it by loading your OS on your own).
Most phones are even like this... Blackberry, android, etc... the only ones where it is FULLY manufacturer controled to download/update the OS, is apple.