If you own the digital rights to your book, then you can publish your book/s on the iPad. You don't have to be published through big name publishers to make it into the iBookstore. Exciting!
Here are your options so far (no doubt more are coming!):
Smashwords. Definitely the most egalitarian distributor out there, Smashwords now has distribution deals with Apple for the iPad as well as Amazon for the Kindle, Sony for the eReader and Barnes & Noble.com. You can load your book for free onto the site but it does need to conform to specific guidelines in order to make the Premium catalogue which includes the iPad, for example, it needs an ISBN. Check this article for how to ensure you get distributed.
Price: FREE (but you will need to spend some time formatting)
Amazon DTP for Kindle. The Kindle app is available on the iPad so if you publish on the Kindle you can still get onto the iPad. Go to dtp.amazon.com. There are lots of help documents!
Price: FREE (but you will need to spend some time formatting – check out this podcast with Joshua Tallent from EBookArchitects for help.
Lulu.com. Lulu is one of the top print on demand distributors and they are offering a way to get onto the iPad as well as offering a free iPad with some of their packages. Lulu is aimed at helping authors self publish and has various packages offering services, for example, if you don't want to DIY formatting you will need some help.
Price: $2399 upwards (not for the faint-hearted!). I am mentioning this because I want to include the many ways you can get on the iPad but as I have posted before, self publishing should cost you nothing.
Create your own iPad app. Most iPhone apps will function on the iPad so if you have one in progress, then it's still a good option. Check out this podcast on how to create your own iPhone app for more info. The developer's kit for the iPad is now available and there will be a truckload of new apps coming out for the iPad soon.
Price: $$$$ (depends on what kind of app you want, but certainly not free!)
Disclaimer: ok, so I need to modify my own ebooks on Smashwords before they get on the iPad. I usually only post on what I have done myself, but this is so exciting, I thought you would like to know!
Update 1 June 2010: You can now self publish direct to the iPad iBookstore but only if you have a US tax number.
I now offer a multi-media online short course on how to publish your book on the Kindle, iPad, Nook and other e-readers as well as answers to all your ebook publishing questions. Featuring video, audio and PDF information for only $39.99.