I have had my 8330 for a couple months now and recently have not been able to put my phone in standby by pressing the standby/mute button like i used to. Also, I know the button is still functioning because i can still mute my ringtone with the button. The only thing that I can think of that I have changed is the theme, so I don't know what could have affected this..thanks in advance.

The button may be wearing down. Have you tried a different theme? Have you tried a hard reset (battery pull while the device is on)?