Activation was pretty much immediate - I was able to monkey with the phone, make calls, etc. right away.
A few minutes after activating, my old phone (via Sprint) quit allowing outgoing calls.
I'm not sure how long it took for incoming calls to be properly routed to the new phone, as I didn't get any on either phone for the evening. (At least, none that I know of. 😛)
One friend of mine had no troubles either (existing AT&T customer). Another friend (coming from T-Mobile) had all sorts of hassles, but it's mostly worked out now.