there is an addon through cydia you can use to help ease the pain. the package is called "close button widget" just launch sbsettings and the toggle should be there.
My wife's i4 has a problem with the lock button it's like really hard to press and she's waiting for the next iPhone. So I jailbroke her device and installed activator and now she locks her device by double tapping the status bar and it works like a champ. That's what I would suggest for now, to replace it involves opening the device and taking a bunch of shit apart to get to the button and replace.