If I may interject here, is it as simple as pressing a button?
The deactivation/activation process a mixed bag that depends on your ROM, frankly.
Yesterday I experimented a little (actually unrooted and re-rooted my phone several times in the process); here's an uncomprehensive look at things:
It seems nearly impossible on any phone with Amon_RA on it to
wipe the activation state of the phone - that is because the "Factory Reset" in the OS Settings area relies on the stock recovery to perform both the wiping of /data
and the wiping of activation data. The Amon_RA recovery will do the filesystem wipe of /data and /cache, but it does not touch activation data.
Granted - your question is about activation, not
de-activation. (The reason that I mention unrooting is that in order to test activation, I wanted to de-activate the phone first - so I had to go back to an unrooted phone to do that).
When it came time to activate the phone, I tried it with a Froyo ROM - Tazz 2.0. I could manually make the call (*228), but could not for the life of me get a dial keypad to appear. (Pressing "Menu" while in the call brought up a menu, one item of which was "display keypad" or similar - but it didn't work.) Argh.
OTOH, after a few rounds of nagging me to press press a key, the VZW *228 call eventually went through the activation process (without me having ever pressed a key!).
To the OP - I would feel a lot more comfortable about selling a phone that had it's activation status wiped. And right now, unrooting and performing a factory reset are the only way to insure that. Granted, if you notify the carrier that you are selling your device and no longer want the ESN/MEID associated with your phone number, you are unlikely to have troubles. But, until you have unrooted and performed a factory reset - your activation data is still in the phone.
eu1
PS
This ROM (XDA) will reset you back to factory, including restoring the recovery partition to factory.