Before iCHat2Cell:

Launch ichat. Select new chat with person or simply hit SHIFT+APPLE+N. Enter +1 and the number you wish to send an SMS to. Enter message and send.

After iCHat2 Cell:

(And after donating a few bucks to unlock the import feature) Launch ichat. Launch iChat2Cell. Scroll through a non-alphabetized and seemingly random list of numbers to get the contact you want. (No way to type the first few letters of a name, etc.) Find the contact you wish to IM (you have to wait a few seconds for the address book to load before you can do this). Enter message and hit send.

You decide which is easier.