Barnes and Noble's Nook reads the same format as the iPad--epub.
Small correction - iPad as a device will read pretty much any ebook format. Last survey I saw said that about 3/4 of the people reading on iOS devices were using Kindle software to read Amazon books (but that information is months out of date at this point, so I'm not sure whether Apple has gained ground back on their own devices or not, yet).
Apple's iBook reader and store uses epub, though.
I would look at the main formats out there and then check out what company would let me do more coverage.
I think with Smashwords, you can do epub, html, mobi, pdf...
It's not just about formats. As Medievalist rightly pointed out, it's about retailers - getting your book into as many retail outlets as possible. That means the Amazon and B&N for sure (about 90% or so of the US ebook market between the two). But Apple is growing in importance in the US, especially if they kick Amazon and B&N off iOS devices June 30th as some folks think they will (I doubt they will, but that's another issue).
Kobo is important too, especially with them planning to invest heavily in the growing EU market. Sony, while still a bit player, is another venue that you can make sales on. Smashwords is pretty tiny for sales on-site, but their use as a distributor (to easily and quickly get to Apple, Kobo, Sony, Diesel, and if you want B&N as well) is exceptionally useful. As more retailers spring up and Smashwords adds them, that utility will only grow. Probably worth putting the book up on Google eBooks, too - even though their market share is very small, it's still just one more store to have your book for sale in.
Formats are easy. Formatting a novel or basic nonfiction book is *exceptionally* simple, once you know how. They key is not in what formats you have handy - it's in what *retailers* your book can be purchased at. The more, the better.