I have managed to find the secret combination of AT commands [1] that will allow you to save an incoming message to the SIM card and then you can read it on the iPad. 

You will need the following commands in order to receive SMS to the SIM card.

at
at+cmgf=1
at+cnmi=0,1,0,0,0
at+cpms="SM","SM","SM"
at+cmgl="ALL"

You can make use of the sendmodem [2] code to make life easier. You can use minicom too!

Example: 
./sendmodem at
./sendmodem at+cmgf=1
./sendmodem at+cnmi=0,1,0,0,0
./sendmodem at+cpms=\"SM\",\"SM\",\"SM\"
./sendmodem at+cmgl=\"ALL\"

Known problems:
There is an awful time out issue. To start this process you will need to './sendmodem at' until you get OK and then quickly send the rest of the commands.


Enjoy!

Refs:

[1] - GSM AT Commands example (good ref really) -http://www.arcelect.com/GSM%20Developer%20Guide%20-%20GSM%20AT%20Commands%20-%20Rev%20%20A.pdf

[2] - sendmodem - http://code.google.com/p/iphone-elite/wiki/sendmodem