Swisscom, Sunrise, and Orange are the 3 swiss GSM operators. Sunrise has the most attractive prepaid rates, Swisscom the least.

All their prepaid SIM's support roaming, but with very unattractive rates, IIRC 2CHF/min in France.

You can buy a prepaid SIM for any of these operators at most electronics shops, including the Interdiscount store at the departure level of the GVA airport (top floor, just across from the emigration/passport control booths).

Swiss GSM cards will expire 12 months after last use.

Refills can be bought in stores, but also at post offices and train station vending machines. If you link a credit card to your number, refills can also be done over the phone.