Will downloading OS4 onto my 3G iPhone prevent unlocking/jaibreaking it later?

I have a 3G iPhone (unactivated) that a friend just gave me. He upgraded to the new iPhone 4. He wiped it clean of all apps and data. It only has the "connect to iTunes" indicator and the emergency call feature loaded onto it. I want to use it like an iTouch for now (download music, apps and use the wi-fi feature) but might want to activate it later using either my T-Mobile account or an AT&T or Verizon plan. If I connect it to my laptop and download the latest edition of OS for iPhones, will it prevent me from unlocking it or jailbreaking it later?

I have noticed that a number of people have experienced problems with OS 4. Is there a way to download an earlier version of the OS for iPhones without using a data plan (internet access) or wi-fi since I have no OS system on it currently and it's not activated.

The details on what is and is not allowed here in regards to discussing jailbreaking phones is a little fuzzy to me. I think that since you aren't asking HOW to jailbreak it this is allowed. If not, expect a mod to either lock or delete this thread pretty soon!

Anyway, I'm about to sell my 3G (8 gb model). I installed 4.0 on it a week or two ago without any problems. I also jailbroke (not unlocked yet) my phone a couple of days ago and have not experienced any problems YET. So to answer your questions, a 3G running ios 4.0 can be jailbroken and unlocked.