Has anyone filed a patent on software?

I want to write an iPhone application and then sell it to a few particular companies, but I don't want to show them the idea and then they just take it and do it themselves. I thought the best way to avoid this would be to patent it, then sell it. Anyone have experience with this? I keep reading online to get a lawyer, but if I can avoid paying for one that would be nice.

In the unlikely event I were showing people software made by me, I would send myself a mail package with a paper print of the source code, several screenshots, a written description and whatever else. If the situation warranted it, like a big project, I would invite a friend to take a look and write a short statement with screenshots and mail it to me. Preferably on several stages of the project, so that I could document the progression.

But as for patents:

http://www.uspto.gov/
http://en.wikipedia.org/wiki/Software_patents_under_United_States_patent_law
http://en.wikipedia.org/wiki/United_States_Patent_and_Trademark_Office

+1 to a patent!

Without one, you do not legally own your idea. If there is money to be made, protect yourself!