I activated my N5 on Ting about 3 weeks ago. They make it very easy. You do need a Sprint Sim card which you can either get from:
1) Ting for $10, (Ships on the 28th)
2) Bestbuy for $30 (most stores have them in stock, this is the easiest way to get one if you're impatient)
3) Ask around at Sprint stores and see if a manager will give you one. This works for some, but it didn't work for me.
I was too eager to wait for Ting to get more in stock, so I overpaid at Bestbuy. Once you have the SIM it's easy to activate. Check out this chicks blog if you have any questions. She was very thorough.