Brendan,
I suppose each person needs to set their own standards.
As far as logging on to a forum in a suspect name,
If its for giggles, i dont see any more harm than that which occurs to a celebrity or politician when someone lampoons them.
Addo correctly describes it....."The assumption of multiple user identities, together with attributed posts that weakly mimic the least serious moments of affected people".
Mimicking is not quite theft.

I'm from a generation where things like log-ons on a computer are considered as private as a sealed envelope (which is perhaps why people dont think twice about leaving me the keys to valuable things), so it came as a huge surprise to me a few month back when I was hanging out with some 19 year old waitresses who were talking about the chaos that they caused each other when they found their mates facebook account logged on.
I was flabergasted that someone would jump to some else's account on its host's computer.
That is identity theft as is your hypothetical.
Creating a log-on in the same name is not.
Its just like those clever dicks who registered 'www....'insert company name' before the rightfull owners cottoned on to the net.
Its not nice and its not polite, but its not identity theft.

Jo