The v0.5 client (still experimental as of time of writing) supports a function where you can sign a text message with a private key.
So you could:
a) have a "register" button next to the address.
b) provide the user a message to sign which includes some identifying information (account name, email address, etc) and the address.
c) user will run bitcoind signmessage ""
d) this will sign the message with user's private key.
Only a user which has access to the private key that corresponds to the address in question will be able to generate the correct signature. Think of it as similar to a Bitcoin transaction but it isn't published to Bitcoin network and involves any text message.
Eligius uses this method to link NMC payment addresses to BTC payment addresses for his pool.
Obviously this will have more value once Bitcoind v0.05 is publicly released.