Ah yes, the reason that you can't figure out how to delete unused or unavailable iOS devices from the Find My iPhone [iTunes link] application is because it's impossible. The only way you can manage the actual list of iOS devices accessible through Find My iPhone is to use a Web browser to log in to your Mobile Me account, then clean up the list from there.

I've also had problems with older iPhones showing up, and when I had a hardware problem with my iPhone 4 and Apple replaced it? Seems like they're seen as different devices by the Find My iPhone application. Makes sense, except it ends up polluting things, as you've already found.

Enough about the philosophy of the service, however, let's jump in and learn how to prune that pesky list so it's just devices you actually do want to track!

I'll start by again showing the list of devices known by Mobile Me / Find My iPhone:

I actually only have two active devices out of this list, so it's definitely time to clean things up. To do that, as I said earlier, I need to log in to Mobile Me:

Once your signed in, you might find that you're dropped directly into your inbox. No fears, just click on the little cloud icon on the top left:

Viola! All the major application areas in Mobile Me are shown in a form that's exactly analogous to pressing Cmd-Tab on your Mac OS X based system:

No surprise, you want to click on the "Find My iPhone" radar screen icon on the extreme right. Because this is a pretty major privacy area, particularly if you share a Mobile Me account, the service asks you to verify your password again:

Typed it in correctly? Sweet! Now you'll see a list of devices known by Mobile Me for your account (e.g. that have logged in at one point or another in the past) on the left side of the screen in the Web browser:

Click on a device that's no longer in service and a small informational window pops up, even as the map behind it becomes a generic white and blue world map (you can just see a bit of it in the background below):

See that "(Remove)" link? That's what we seek. Click on it...

If you're sure that it's a device you want to delete from the list, go ahead and proceed. Frankly, even if there's some confusion or fear of making a mistake at this point, the worst possible case is that you accidentally remove a device you wanted to keep. No worries. The first time your device checks in again it's automatically added to the list!