How do I make sure my blog is iPad / iPhone / Android / touch-device friendly?
I personally use the WP Touch plugin on my site. It automatically detects if the user is browsing with an iPhone (or iPad) and reformats the site into an iOS-friendly layout with touch-sensitive controls. There's an option at the bottom for visitors to display the normal site if they want.
It's a very well-supported system, and I've used it on 3 of my own sites and 12 client sites so far without any issues whatsoever.
Alex King's WordPress Mobile Edition plugin is my favorite:
The most iPad-unfriendly thing you can do is try to cater for the device with a dedicated theme, especially if that theme is the OnSwipe one that WordPress favour. Please do not do this unless you actively want your iPad-using readers to hate you.
If your blog is WP-hosted, this will be enabled by default. Please go to your dashboard and turn it off.
I would also recommend WPTouch which now has great iPad support. There is a basic free version which EAMann mentioned but I would recommend the paid version which is only $39 and has a lot more features. You can then customize the look and feel.