First, you should know that TextEdit defaults to the .rtf format. If you're not familiar with it, RTF is "rich text" and, unlike the .txt files generated by something like NotePad, RTF includes formatting, like bold or italics or bullet lists. "Plain text" .txt files are pretty much just the basic ASCII characters and paragraph breaks. So what? Well, if you want things to look pretty, you'll stick with .rtf, a format which is easy to share across platforms. Side note: did you know TextEdit will open Word documents? It isn't perfect, but it works if you don't have Word on your machine. The older .txt format is better for coding or when you don't need or can't have formatting.
To create an inbox, I suggest the simpler .txt format. What I used to do was set up Quicksilver to easily append to an inbox.txt file, and I used GeekTool to pin that .txt file to my desktop. You could also use LaunchBar to append, and I'm sure there's a way to whip up an AppleScript, but I never bothered. Instead, when I ditched Quicksilver, I started keeping the text file in the Dock, and I just open it up to add items. All this is portable, indexed by Spotlight, and fully cross-platform compatible.
Next up: munging HTML with TextEdit, and grabbing snippets of text from any app and dropping them into a file.