Depends on whether you were banned simply by the server owners, or if you were issued a global punkbuster ban. If the latter, your GUID is permanently banned on any server running punkbuster. Punkbuster doesn't issue those lightly - they do so only after you are found guilty of cheating based on a complaint from a server owner accompanied by PB screenshots. It is extremely rare that Punkbuster would reverse a global ban once they issue it. If that's what it is, you're hosed as far as your current game goes - you'd have to go buy another COD4 license so you'd get a different GUID.
If you were banned from individual server owners, you're only option is to go to them individually and appeal the ban.
In neither case is there any way to "unban" yourself.
Server owners and admins (I am one) generally have a low tolerance for ********s and ZERO tolerance for cheaters. If you were banned from several individual servers, perhaps you should sit back and realistically evaluate your AQ (Asshat Quotient).
