Registering depends on where you live, obviously. However, I would have to agree with Kreed on this - they know who is buying them. Just because you don't pay a registration fee doesn't mean the transaction is not recorded or even sent to the ATF or anyone else for that matter.
In Oregon, any transaction is recorded and all sales are 'registered', so long as they are done by a licensed seller. This means that any store, or FFL dealer selling you a gun will have you complete that questionnaire, pay for the firearm, and run a 5 minute background check. I can't recall at this time if there's a waiting period on handguns, but you can walk out 5 minutes later with a rifle. ALL OF THESE SALES REGISTER THE GUN IN YOUR NAME.
Any PRIVATE party sells - out of the newspaper, want ads, etc, are not required to be recorded. Since it's being sold out of a garage, there's no background check or anything else. Sure, basic rules follow, don't sell to people under 18 for rifles, under 21 for hanguns. Then there's the common sense things like don't sell to some shady character. However, the point is these sales are LEGAL in Oregon and they are not recorded, tracked or otherwise registered. In addition, you don't need to register it. The only real exception to this that I can think of is if that transaction crosses state lines, then it must go through an FFL and be recorded as entering/leaving the state.
As for criminals, I have to agree - most don't obtain them legally (going to wal-mart and buying a gun) and most are probably stolen or bought from the 'black market'. So making it harder to buy a gun does very little in affect to controlling criminals. Another thing - the UK has pretty much banned guns altogether - those that aren't are extremely difficult to get. Most crimes happen at knife point or with baseball bats or other weapons. The fact is, if someone is going to commit a crime - they are going to do it regardless of what weapon they have at hand.
As for Rosie - she's a moron. She used to be the spokesperson for Kmart. She had Tom Selleck on her show once and chewed him a new ass because he was a spokesperson for the NRA. He simply explained that he grew up around guns, supports the right to own them and so on. Howard Stern played this on the air and then proceeded to dog her because she bitched out Tom Selleck for supporting the NRA - yet she supported Kmart which sold guns. She later quit her contract with Kmart. Go figure.
Bottom line - I would guess that if you filled out that 'little form' when you bought a firearm - someone in the government knows you have it.