I'm doing the same thing -- selling my 4S and using my iPhone 2G in the interim. Tonight I made my own micro-SIM to SIM adapter from an expired gift card (which was made of the same kind of plastic stock as a credit card). Once I made the adapter, I took the micro-SIM out of my 4S, stuck it in the adapter and put the result in the iPhone 2G's SIM tray. It worked perfectly. You should know, however, that my iPhone 2G is unlocked.
So yeah, in short the 4S SIM works just fine in a 2G as long as you have an adapter, which you can make yourself if you don't want to buy one on eBay and wait for it to ship. Here's a pretty basic article to help you out:
http://www.jonathansaggau.com/blog/2010/06/diy_iphone_4_microsim_adapter.html
I advise putting a piece of scotch tape or packaging tape on the backside of the adapter to help hold the micro-SIM in place.
You will be able to make and receive calls just fine on the old 2G, as well as use EDGE data connectivity if you can possibly stand it. If you were using iMessage on your 4S, I noticed that continuing to text people is not quite seamless. I can text my friends from the iPhone 2G and they receive normal SMS messages right away, but if they send a text back to me, it still gets sent as an iMessage and I don't receive it. They have to tap and hold on the message they just sent me and then tap "Send as Text Message" when the prompt appears. Then I get it on the 2G as an SMS right away.
Hope this helps.