If you want to see that "Run a Text Command in Terminal" action as duffbeer703 shows, you'll have to add the Terminal plugin.
**Sorry, I didn't see that you wanted to run TextEdit instead of vi. In order to do that, do this:
Make a ~/bin folder (that's a folder named bin in your home directory).
Add ~/bin to your path by editing ~/.profile and adding the following two lines:
Go into ~/bin and make a new file called TextEdit with the following line:
sudo /Applications/TextEdit.app/Contents/MacOS/TextEdit $1 &
Make that file executable by doing: chmod +x ~/bin/TextEdit
Now go back to QuickSilver and its "Run a Text Command in Terminal" thing and do the period thing to type in text, then type: TextEdit /etc/php.ini
A terminal will popup and ask for your sudo password. Once you put that in, TextEdit will pop up and let you edit as root.
There's probably an easier or cleaner way, but it does work.