There is in the App Store a free and a pay Siddur for your iPad to make it davening friendly (search for Pocket iSiddur and Siddur HD respectively).
We have a better suggestion. Here it is in a nutshell.
Buy GoodReader (99 cents). It is just so good the we recommend it for many uses. We had the Rinat Yisrael siddur (PDF) saved in our Google Docs. You can just go and get yourself a copy here http://hebrewbooks.org/34229 (or get any of a number of prayer books from that site in PDF format) and then open it in GoodReader and you are done. You have the siddur that you like and can read it in portrait, landscape or 2 page landscape. It pages smoothly and reads clearly.
Besides this one off use, the GoodReader app restores to you the sense of control over your files on your iPad. It's good!