Depending on your browser, there may be no sane way to type tab characters into a textarea.
Alt-keypad0-keypad0-keypad9 might work, or you'll have to copy and paste an existing tab.
This matter is covered in detail in TipForTypingTab, which should be read before voting below.
The EditPage should have a JavaScript tab-inserter, or at least a 'click-here-to-copy-a-tab'-button. Or a small text box where one can type the character sequence to be globally replaced by tab when Save is clicked.
I suggest a vote (add Y to agree, N to disagree): [YNN]
How about moving "I can't type tabs" up, close to the Save button? -- BenTremblay
JavaScriptIsEvil?. Don't make me enable JavaScript just to edit wiki. I would far rather have all dependence on tabs removed from the formatting rules.
See also: TabMunging