I know a few people who carry two mobiles around with them, one for personal use and one for work/SJA. So, suppose that you could fit two separate SIM cards into the same physical phone. For incoming calls, both SIMs would make the phone ring; you could assign separate ring tones if you wanted to (as you can now for particular caller numbers), and it would handle simultaneous calls in the same way that current phones do (e.g. beep and say "call waiting"). For outgoing calls, you'd have a slider control on the side of the phone to select the SIM (I have something similar on the remote control that handles TV/DVD/VCR), and the call would be charged to the relevant bill.
Anyway, feel free to steal this idea, royalty free. There may even be scope for a "3 SIM" phone in the future, in the manner of those razors that use multiple blades. How many is just too damn many? We may never know.