Consider the following scenarios:

A. I install Debian Squeeze (or more generally, Debian Testing after it has been frozen) right now and keep it up to date until the day it becomes stable.

B. I wait until Debian Squeeze becomes stable and install it (and update right after installation, of course).

My question is: Assuming that I do not do any "hack" to the systems in either A or B, is there a difference? Will A be equal to B?

I guess there will be "cosmetic" differences. But other than that, will they be exactly the same in every other aspect?

Yes, it is likely to be the same, though you cannot in general guarantee this.

What can change is the defaults configuration for an installation; subsequent updates may not change those settings, whereas a new installation will.

A hypothetical example would be the default filesystem (an extreme example, because it isn't the sort of thing that would change after the freeze). An ext3 install will not be automatically upgraded to ext4, whereas the final install could have the default changed to ext4. Similarly, if you are running the old version of grub, updating will not automatically install grub2.

One would imagine that most configuration changes would not fall into the 'release-critical' category, so you aren't going to see major changes like the examples above between now and and the final release. But it isn't impossible.

I just started using(well installing) Debian when it was 5.04 Kenny stable and I haven't been able to enjoy it due to my being a linux newbie(didn't know much about windows either). I found it easier to install Squeeze than it was to install Lenny. I haven't been under the hood because I'm still trying to stablize the Ubuntu 10.10 installation on my other machine. I guess after this I'll work on networking.