Does the wallet not need to go online to be "validated" by the network?
Nope. Anyone can send coins to any address, even if it has never been seen on the network. The address includes a checksum to prevent typos, but other than that there are no restrictions and no verification that anyone actually has the corresponding private key.
I know this is a remote possibility, but can't the wallet generate a receive address that is already in existence?
Yes, but the probability is absurdly low. If every computer on Earth was dedicated to generating keys it would still be extremely unlikely to have any collisions before the sun explodes and destroys everything.
Which wallet gets to control the btc attached to that address then?
In that case (which will never happen), either person would be able to spend any coins sent to that address.
I will create an offline wallet with client 0.5.0.
I suggest you use a paper wallet generator. It gives you much less data to store and the secret key import format will be much more forward-compatible than relying on a future version of the client to open an old wallet.dat.https://en.bitcoin.it/wiki/Paper_wallethttps://www.bitaddress.org/