OK, I found this out when I updated to the Mac 10.6 Snow Leopard OS. It changed all my text documents to open up with Apple’s TextEdit instead of my usual BareBones TextWrangler program. The fix is simple, but it is listed nowhere that I could find.
Go to one of your documents and Right click (or CTL Click) to get the menu that pops up to the side. Choose to “Get Info” from the list. In the box, there is an “Open With” drop down box that will allow you to choose whatever program you want. Choose your program. Underneath that box, you’ll find a “Use this application to open all documents like this one.” Note that you’ll have to choose the program before you get the “Change all” button to show up.
When it does, click it and you’ll get a window that basically says “Are you sure you want to change all similar documents to open with the application “TextWrangler.app”? This change will apply to all documents with the extension “.txt”.
Click “continue” to make it happen.
This should work with all other programs as well.