I purchased my 3GS used while abroad. I know it was carrier locked and have a good idea it was AT&T (United States) since their the only official carrier. However, before I have my friend send me a pre-paid SIM is there any way to figure this out?

I'm doing it as a precaution against future activation issues. Thanks!

What will be important is to find a reliable local source to update your phone for you. I had an unlocked I Phone and found that I could not install the Apple software versions via I Tunes . I needed to go to my friendly mobile phone shop and they would install the Apple OS updates.

Because the OS updates are not released frequently it is not really a hassle and in fact you don't need to keep updating the OS unless you want the latest features.

I'm not sure why you're asking the question. What kind of pre-paid sim from which country and for what purpose?

What you can do as mausplan said above is you can go to a shop and have them jailbreak/unlock it for you. Compare prices first, some overcharge for this service. All the software to do this is available free but there are many pitfalls and you really have to know what you're doing so I'd not recommend doing it yourself.

I jailbreak and software unlock with Pwnage Tool IPSWs / Ultrasn0w, it's okay there. Since Ultrasn0w supports all the basebands right now, that's also okay. It's more of a safeguard to prevent this from happening (noted though, all of my jailbreaks and unlocks since 3G and the original software, it's the first time this has happened) again, assuming not all basebands can be unlocked. On my last update from 3.1.3 to 4.0, I ran into a corrupted baseband (I posted my fix somewhere in this forum) which nearly bricked my phone. Long story short, the IMEI was different than my SHSH file, fixed them and flashed the baseband. Manually installed files from 3.1.2 and tried to set them through mobileterminal. Next ended up having to upgrade my baseband. I could restore, as long as tethered jailbreaks exist - or if not (seeing the way of the web based jailbreak) just to get past the emergency screen as long as I had an original SIM (or I saw somewhere a phonebook SIM will do the same).Long story short, it's just to avoid a potential problem in the future. I'm nearly certain it'd be a pre-paid SIM from AT&T in the United States. Also, I suppose now I'm just curious.

I believe you could run without SIM, turn on WiFi, and run the web based jailbreak over WiFi. I don't think you never need the original SIM...

http://www.jailbreakme.com/