By touch screen, I mean things like the iPhone and iPad.
How are touch screen devices being made accessible? Are there any development guidelines that should be adhered to? Could a blind person ever use an iPad successfully?
I think Apple puts a lot of pressure to making their touch-based screen devices as accessible as possible. As far as I am concerned, iOS has pretty good set of accessibility features including voiceover, support for aiding hardware and so on.
ATMac, website dedicated to disabled people using Apple products, once published a pretty good round-up of assistive technology in the iPad and Royal National Institute for the Blind People in the UK published a review of the iPad from a visually-impaired user perspective and as far as I remember, it was pretty good.
iLounge recently published article about accessibility improvements that are coming in iOS 5 that includes adding support for specialized hardware accessories created for users with mobility, hearing, vision, and cognitive disabilities as well as LED flash and custom vibration settings to signal when people are calling. Apparently, it also includes VoiceOver improvements such as custom element labeling to provide personalized spoken cues for menu options.
As Mariusz said, Apple puts a lot of effort into the iOS SDK to make it easy for developers to have their apps as accessible as possible. For example, if you're using standard label or text controls they can be read out loud by the OS.
Matt Gemmell is a software developer. He has come close to beginning to lose his sight at age 31, and wrote a fantastic article about iOS accessibility that I'd strongly encourage you to read.