I use this one and like it. Daniel Ethier's Gradekeeper is also good, but the sync between the desktop and the iPad is kludgy.
OFP, I hope you find what you're looking for. There is also an iPad app called TA Attaché that I have, but haven't really put through its paces yet--I'm not sure if that works on iPhone or not. I don't think either Gradekeeper or Gradebook Pro works on iPhone.
And I luuuuuuurve my iPad, so you may change your mind about the cost/benefit ration once you've dropped about $100 more in the iPad app store. I rarely use my laptop now.