I have recently been using the modules from RFDigital and they are incredibly easy. They use the phrase "Wireless Pipe" and it really is like that.
The base-unit is an SMD module of 15mm (0.600 Inch)square, 3.5mm (0.125 Inch) in height so it looks as if it would fit on your board profile. As it is a 3.3V module it would obviously require a regulator for the supply rail and level-shifting in addition to the module itself, but that should also fit easily..
It comes in 2 'flavours' - in-built chip antenna (RFD21733) and external antenna (RFD21735) both of which operate in the 2.4GHz band. The RFD21733 (probably the most usable for your range of products) is CE, IC and FCC APPROVED.
I think that a board using these would be a good addition to your range.
It was originally intended that I would be writing application notes for for RFDigital, for these and their "Wireless Inventors Shield", all of which would be using the Arduino (which is why I have samples) but that never happened for various reasons. I therefore have a set of outline application notes to support the product if you wish to pursue the idea. Looking at your range, there seem to be several units that would dovetail with RF very neatly to further expand application notes.
It would be nice to see this materialise as a product and, obviously, I'd like to receive "free stuff" (who wouldn't) but my main reason for suggesting it is that there are one heck of a lot of people who believe that efficient RF Comms with Arduino must involve an X-bee, with all its complications, and that is simply not the case.
These are easy to use, can operate in UART mode (use standard Hardware Serial - RxD0/TxD1 - or Software Serial) or three-channel Switched Logic mode with selectable output durations of 20mS or 500mS and can operate in broadcast or network configuration (with a "learn" mode). Most importantly, they work - very simply and efficiently...
I should say that I no longer have any connection with RFDigital, I just believe it's a great product that deserves a wider user base within the Aduino community and I think it would marry well with the Tiny range.
Duncan Amos