http://www.last.fm/help
Theoretically, I don't think you'll need the Mac software to scrobble songs (though it helps!). You just need a Last.fm profile, and then you can access the various Last.fm radios through the iPhone app, and your songs will be scrobbled then.
However, I think it's best when you scrobble your own songs from iTunes -- that way it builds a more accurate music profile for you. Be warned, it can take a LOT of scrobbling before you start seeing the benefits -- but once you do, it's definitely worth it. I've discovered SO many new bands that are my style from Last.fm, while also making tons of friends who listen to similar stuff (not just casual internet friends, but really close friends I regulary do stuff with). Plus, I've been connected to many concerts I otherwise would have missed. It's for serious music lovers, and when it works, it works beautifully.
Pardon me, but are you scrobbling at the moment?
Thanks for the help and explanation.