Dictionary.com app for iOS comes with voice-to-text support
This works pretty much as you'd expect -- you hit the new Voice tab, tap the screen and speak the word you're looking to define. If the app finds multiple matches for what you've said (which usually happens when it doesn't fully 'understand' what you said), it will show you a list of those allowing you to choose the one you've been after. According to Mobiputing, the system works quite well, even for longer words.
However, there's an interesting caveat here: the voice-to-text functionality is only free for the first 15 voice searches you perform, in both the free and the paid app versions. After you've reached that limit, you have to purchase batches of 300 voice searches, and each one of those requires you to pony up an additional $0.99.
Other features of both apps include sharing definitions to Facebook and/or Twitter, viewing the word of the day and question of the day, and favoriting words. There's also a Thesaurus section that lets you find similar words, and of course you can also define words by doing the old-fashioned thing and typing them in.
Dictionary.com for iOS is compatible with iPhone, iPad and iPod touch and requires iOS 3.0 or newer to run.
Download Dictionary.com for iOS free from the iTunes App Store
Download Dictionary.com for iOS paid from the iTunes App Store