its up to you.. how much money do you want to spend?
you can get a curve now for pretty cheap and its a great phone.. or you can wait for the niagara (not a curve) to come out and you will spend quite a bit more..
If it's your first blackberry I would go with the curve. It's an amazing, and reliable phone. I was going to wait for the storm to come out. But now I see all the problems people are having... I wouldn't want to be part of that I'll stick with my curve for a while yet
I just got my Curve and haven't regretted the purchase yet. I don't see any real issue when the networks merge as far as your phone is concerned since they are both CDMA phones.
I suppose the biggest booty-kicker would be color choice. Verizon and alltell both have the Titanium colored ones but if your looking for the pink one, then waiting on verizon would be your choice
Well it all depends if you want to lock into Alltels plan for the next 2 years or Verizons plan. If you sign a contract with Alltel before the merger completely done in your market then you will keep the Alltel contract for the duration of it.
Posted from my CrackBerry at wapforums.crackberry.com
Alltel isn't going to be Verizon in the next month...I've heard of different areas being a part of the billing conversion sooner. I also haven't heard of Verizon announcing an exact release date on the Niagra.
As far as the Curve, its a great phone. If you just want one and not renew your contract, look on ebay or CL for one. Or there is the marketplace in the forums here as well.
I'll stick with my curve for a while yet 
