With ICS my phone now ALWAYS mounts my SD card as an external drive when I plug it into a computer. Sometimes I just want power, especially if I am trying to play a game that's on my SD card. Under the previous version I had the option to connect JUST for charging, without talking to the computer at all. Is there a way to get this functionality back w/ an ICS device?
With Android 4.0 ICS, charge only mode is no longer available.
Unfortunately there's no app to deal with this issue AFAIK.
You can try, but I believe that this isn't possible with your phone, but for future readers, placing the phone in "media transfer mode", allows access to SD cards on both phone and computer.
Further into this matter
One can always disable the "automount" feature from the host OS:
From your home screen go to your settings. Depending on the phone ( I have the HTC EVO Design 4G for Boost Mobile) you should see an option for "Connect to PC". Select this option and you should see two things. The first is "Default Connection Type" and the second is "Ask me". Ask me will have a box beside that you would check. Once checked and you exit back tothe home screen any time you connect to your PC itwill bring up the prompt again.
You can plug the phone in and charge it using a powered USB hub that's not connected to a computer, this will just charge without mounting drives.
It sounds like something you probably won't have handy, but have a look at your PC's screen. Many modern screens have powered USB sockets, but aren't actually connected to the PC (unless you plug a second USB A->B cable between the screen and PC which many people never do).
I charge my phone at work this way by plugging it into the screen, as we're not allowed to plug un-approved USB devices into our PCs (and if we do they're actively blocked).