f5 in king's gambit

One of my teammates employs quite an unusual line against the Nf3 king's gambit. Namely, this:

When he first told me about it, I was rather skeptical, but looking briefly at some lines with Rybka convinced me that it is certainly sound. My instinct in the position would be to play d3 and continue developing. exf5 doesn't achieve anything and e5 leaves the e-pawn exposed to d6/Qe7 ideas.

I am not an expert in king's gambit, so maybe this line is not as rare as I think, but I am simply curious :)

So, my question is - do you have any experience with this line, any games to show, theoretical comments, etc?

I know that there are a lot of king's gambit enthusiasts on this site, so I am hoping for some input :)

http://www.chessgames.com/perl/chessgame?gid=1079737

http://www.chessgames.com/perl/chessgame?gid=1278856

http://www.chessgames.com/perl/chessgame?gid=1076683

I'm no good at chess however, so I won't go into deeper analysis.

It's rare, it's not bad (Black has many, too many good answers to the King's Gambit) but 2...Nc6 3.Nf3 f5 is an improved form of that idea.

But I agree with the OP that 4.d3 is the proper answer - actually for both of them.