Iphone Serial # 940 + Not Unlockable?

[Brookstone]

Need some expert advice here.
Pretty much have made decision to get an Iphone. Looking for 3GS 16GB Unlockable to be used with any carrier.
Went to a store selling for $560, but locked to Rogers. Told me that the new 3GS Iphones with Serial #s 939 and under can be unlocked to be used with any, but 940 and up, is not able to. If you put another carriers sim card in, everytime you turn phone off, it resets/erase or whatever what you have done previously. So basically have to keep unlocking it everytime you turn off and on. Which I did not know that before....which is now hindering my decision. But then I also see ads such as this:
http://toronto.en.craigslist.ca/tor/mob/1642434731.html

He's not mentioning anything about any serial #s, so wouldn't that be the same thing as me getting that Rogers phone and get it unlocked? This has got me kind of confused. Some expert advice would be appreciated.

I assume no stores will be selling any 939 and lower models anymore, only new ones?

 

[Hightop]

The I-Phone now comes unlocked from the factory. However once you activate it it locks up to whatever carrier you are using thru I tunes

 

[Brookstone]

I understand that. Pretty much what he mentioned to me right.
So if I buy that phone that is locked to Rogers, and I decided to switch to Fido or others. I would encouter the problems he mentioned or no? For any that is 3GS #940 and above. Because if so, how does the ad I linked above make any sense. It's the same concept right.

Cuz I am thinking of switching to Fido from Telus prepaid, what I have currently. I know I can just order straight from Fido if I do decide to go with them, but if I can find a cheaper deal offsite, that's even better right.

Edit: To be clear. I am asking in case of things don't work out. Say I do go with Fido, but decide to switch to another after a while. Since I'd be locked to Fido anything 940+ would give me problems?