In cell phone land, a SIM is not a Simulation (nor a simulated citizen), but a Subscriber Identification Module. Your SIM identifies you (of course); if your phone breaks, you can move the SIM to a new phone and you're back on the air (literally as well as instantly). But SIM cards are the hallmark of GSM, the Global Subscriber M????. GSM is one of the two major modulation methods in use for consumer PCS cell phones, the other being CDMA. CDMA is used only by some North American carriers, such as Sprint in the US, and Telus and Bell in Canada Most of the rest of the world, and most other major North American carriers, use GSM. Oh, there are a couple of others like TDMA still in use, but you're not going to see any new growth in those technologies.
And GSM is more widely used worldwide. There are four GSM frequencies: the older 800 (Europe) and 900 MHz and the newer 1800 (Europe) and 1900 (North America) MHz. My older Treo 180 was dual band (1900 and 800) so I got some coverage on my last trip to Europe. The Treo 600 is quad-band; it covers all four frequences, so just about anywhere you go you should be able to get coverage (assuming your home carrier has roaming agreements there).
CDMA (and TDMA) connect your subscriber identity to the electronic serial number that is burned into the phone. GSM connects your identity to the SIM card.
Sim cards start out life as credit-card-sized Smart Cards. After activation, they are punched out to a much smaller size, which fits inside your phone.
Since carriers pay the phone makers a subsidy, they get in return a lock on use of the phone. Locked GSM phones will not accept another carrier's SIM; locked CDMA phones cannot be activated by another carrier.
Most people are happy to sign a 2- or 3-year contract with a single supplier, in return for the $200 or so they get off phone compared to the "no-contract" price. As of April 1, 2004, according to Handspring's no-carrier page, for example, the Treo 600 sells for US$599 for Sprint, US$699 for GSM. The GSM phone is obviously unlocked, since there is no carrier specified. It isn't clear if the Sprint phone is carrier-locked or not. Other CDMA carriers like Bell Canada or Telus wouldn't know the sequence of 'secret' keystrokes to activate a Treo 600 anyway, so the question is probably moot. At the same time, according to their Treo 600 overview page, you can buy the phone with activation for US$449 (including free shipping).