If by abroad you mean you're moving from the US to elsewhere, it's possible you can use it for international use.
The iPhone 4S is a true world phone that is unlocked for international use at your previous carrier's discretion.
Both the CDMA and GSM phones allow you to place in a GSM SIM Card from a European Carrier, for example, and use their network.
Not by default. Firstly, iPhones sold through AT&T will only ever accept AT&T SIM cards, even if you travel overseas. Only unauthorized "hacking" can change that.
Secondly, iPhones sold through Verizon or Sprint will, by default, only accept special SIM cards, carrying Verizon or Sprint branding (respectively), whose sole purpose is to connect to their specially negotiated roaming partners internationally. (There was apparently an exception, where the first several shipments were delivered without this restriction - but they are the exception rather than the rule.)
If you are a customer in good standing for a certain minimum period of time, then you can pay a fee to Verizon or Sprint and they will remove the restriction on international SIM cards, so that you can roam internationally using local SIM cards rather than being forced to use the special Sprint or Verizon-branded SIM cards. However, as soon as you travel back to the USA, these phones will recognize the fact that a native CDMA service is available, and they will go back to only working on their home Sprint or Verizon services, regardless of whatever 3rd-party SIM card you may have installed.
So, unless you're willing to perform unauthorized hacks, the summary is:
1) AT&T customer? AT&T SIM card only, both domestically and internationally.
2) Verizon or Sprint customer?
2a) Native CDMA signal detected? SIM card is totally ignored (under default settings), whether you're in the USA or in one of the handful of foreign countries where Verizon/Sprint's roaming partners are CDMA-based.
2b) No native CDMA coverage detected?
2bI) Not unlocked? Verizon or Sprint SIM card only, for international roaming. Back to Verizon/Sprint CDMA service (under default settings) as soon as a native CDMA signal is detected.
2bII) Is unlocked? International SIM cards permitted, for domestic rates in each of your international destinations. Back to Verizon/Sprint (under default settings) as soon as a native CDMA signal is detected.