This has always been one of my big needs on the iPhone.
There are a few there now:
$9.99:
SplashID: well known from Palm and other platforms
Mecrets: only two reviews for this one so far.
$4.99 Firebox
I'm waiting for either 1Password or Ilium's eWallet. I've used eWallet for years on Palm and PC but I've purchased 1Password desktop app thru MacHeist so I may have to stick with it.
Through the years I have heard only great things about SplashID(or any of their products) and eWallet. But since eWallet isn't available for the iPhone yet, I'd recommend SplashID if you are in a hurry.
The amazing application of 1Password that has been on the Mac and praised by many is coming to the iPhone FREE of charge!!!!! It syncs up with 1Password and well go to the link above to see screenshots and how the app works - but don't pay anything right now a better app is coming in a week or 2!!!!
SplashID has the big advantage that it exists on pretty much every conceivable platform. If you ever switch to, say, a blackberry, you can just export and reimport all of your info rather than having to retype everything.
SplashID also has a desktop version that you can use to sync your data with.