There is just one plist. It may be possible to specify a different value for the iPad for that key but I'm not sure if it's supported for that key.
It doesn't sound like a huge deal really. You could just remove it from the plist and set it programmaticly. It looks like if you try to set it on an iPad it just sets the status bar to black, so there's not too much reason to even check the UI idiom....
Anyway, see the Information Property List Key Reference docs if you want to try it.
  In iOS 3.2 and later, applications can
  designate keys in the Info.plist file
  as being applicable only to specific
  types of devices. To create a
  device-specific key, you combine the
  key name with some special qualifiers
  using the following pattern:
  
  key_root-~
  
  In this pattern, the key_root portion
  represents the original name of the
  key. The  and 
  portions are both optional endings
  that you can use to apply keys to
  specific platforms or devices.
  Currently the only platform you can
  specify is iphoneos.
  
  To apply a key to a specific device,
  you can use one of the following
  values:
  
  iphone - The key applies to iPhone
  devices. ipod - The key applies to
  iPod touch devices. ipad - The key
  applies to iPad devices. When
  searching for a key in your
  application’s Info.plist file, the
  system chooses the key that is most
  specific to the current device. For
  example, to indicate that you want
  your application to launch in a
  portrait orientation on iPhone and
  iPod touch devices but in
  landscape-right on iPad, you would
  configure your Info.plist with the
  following keys: