Just a quick comment on the iPad and USB MIDI devices. Some USB MIDI interfaces are not fully compliant with the MIDI specification. The most common issue we have seen (and others have reported) is that the interface will not properly handle SYSEX messages. The failures range from ignoring/dropping the SYSEX messages, corrupting the data, and in some instances "crashing" the USB MIDI interface when SYSEX data occurs frequently. Some seem to work "OK" as long as the SYSEX commands do not happen too frequently. This latter type of unreliability can be troublesome because it may look like there is an issue with your controller or Reason when in fact its just an occasional MIDI interface error.
SYSEX may seem like something you can avoid when using the iPad to control Reason, and this is true if what you are after are faders, knobs, and buttons to map to Reason controls. But if you wish to display information from Reason like track names or other labels SYSEX enters the picture. For example the Lemur's "LCD" display widget uses the Mackie Control's SYSEX commands to receive the text it displays.
If you have any plans to use the iPad to control hardware synths and the like then SYSEX becomes even more essential. Many synths, especially those made in the 80's and 90's, rely on SYSEX instead of MIDI Control Change commands (CC) to send and receive parameters. For example Yamaha's DX series synths only use SYSEX.
By the way this is not an iOS specific issues. The USB MIDI interfaces we have tested that failed with the iPad also failed when used as a MIDI interface for the computer.