Yes, the iPhone/iTouch is amazing. 100,000+ Apps. A lot for free. There are a few diamonds in the ruff, and this one is looking like another find.
RelativePitch is an an iTouch/iPhone app that will drill you on intervals. There is a lite version of it as well. The interface is OK, but I have a few complaints about it. The application does not, at least in the lite version, allow you to vary the root note. Maybe the full version does this, I dunno. Second, the intervals are always UP, as in root then the note above. Down would be a great thing to have in this app as well. Perhaps the full version does it. It would be simple enough to add in an update.
The price? $7.99. Forget that. If the developer knocked it down to $1.99, maybe. You’d be better off searching the web for various free trainers, like http://www.trainear.com/. Hopefully someone will do a better version for the iTouch/iPhone soon. Or a cheaper version. $7.99. Come on people!