[url=http://www.law.emory.edu/11circuit/aug2000/99-13906.opn.html]US v. Nelson[/url] sums this up quite nicely, by actually quoting from the 4473:
[B]WARNING - The Federal firearms laws require that the individual filling out this form must be buying the firearm for himself or herself or as a gift. Any individual who is not buying the firearm for himself or herself or as a gift, but who completes this form, violates the law. Example: Mr. Smith asks Mr. Jones to purchase a firearm for Mr. Smith. Mr. Smith gives Mr. Jones the money for the firearm. If Mr. Jones fills out the form, he will violate the law. However, if Mr. Jones buys a firearm with his own money to give to Mr. Smith as a birthday present, Mr. Jones may lawfully complete this form. A licensee who knowingly delivers a firearm to an individual who is not buying the firearm for himself or herself or as a gift violates the law by maintaining a false ATF Form 4473.[/B]
If you pay for it and give it away, it's a gift. If some other party pays for it, and you fill out the 4473, it's a straw purchase.
The only exception to this is that parents may legally fill out the 4473 for their minor children--example, your 16 year old son has been cutting lawns all summer to buy that new AR. Your son pays for it, you fill out the 4473, and your son takes possession of it with your [b]WRITTEN[/b] permission.