I've normally liked to unlock then upgrade, to have the stock iphone firmware but unlocked.

I tried this with pwnagetool for 3.0, but it doesn't work anymore, now it 'deactivates', when it didn't happen before.

Is the same happening with redsnow or can I upgrade to 3.0 again with iphone remaining activated?

anyone? anyone? bueller?

I don't really understand what you mean.
You wrote that you liked to unlock then upgrade? So what does "unlock" here mean?

By the way, I would like to mention about some special term of the iPhone:
-Unlock: mean to do something to the baseband in order to use your iPhone with any simcard. If you use 2G iPhone, bootneuter will help unlock your phone. If you use 3G iPhone, use Ultrasnow instead.
-Jailbreak: mean make it possible to access the system file of the iPhone. Once you can access the file system, you can, in theory, do anything you want on the iPhone (for example Activate the phone).
-Activate: mean to by pass the "connect to iTunes" activation screen. after activation, you can use the iPhone to make call, play games, surf the web...

yes, that's what I meant.

On 2.x, whenever I've had to restore, i've pwned it, then performed a shift-upgrade to the same version, to have the stock firmware (jailbreak always make it sluggish, IMO) but unlocked and activated. Any new fw version I would just upgrade and iphone would remain unlocked and activated.

With version 3.0 it worked, but since youtube didn't work, I decided to create custom 3.0 firmware with pwnagetool to see if it would fix it. youtube worked, but OS seemed very sluggish, so I decided to upgrade to stock 3.0. This time iphone remains unlocked (carrier and bars appear) but iphone is not activated (emergency screen). Now I cannot downgrade, but that's another issue.

I restored version 3.0, ran redsnow, but I'm curious if this activation will hold if I upgrade.

I'm not sure but there is more ability that the phone will need to be reactivated if you update to the next OS (3.XX)
Last year I upgrade my iPhone from 1.1.4 to 2.0 by restoring a custom firmware. Then when 2.1 was out , I UPDATE via iTunes and the activation remained. But then for some reason I reset the phone (press Home and Wakeup button for 10 sec) and after restarting it brought me to the "connect to iTunes" activation screen. I had to run quickpwn to rejailbreak and activate it.
Since baseband have not changed since the release of OS 2.0, update (or restore) will keep your phone unlocked.