I am getting an iPhone 3G on a 2yr contract, come July 11th. I will use that as my primary phone and will use official applications purchased through the iTunes store.
I am also getting a 1st gen iPhone, second hand, possibly Monday or Tuesday. I am getting the 1st gen iPhone to use as a development device and to play with. So, it will be unlocked. I will use it as a phone until the 3G one comes and then the 1st gen phone will not have a SIM, and I will use it for development purposes.
Are there issues that I might have with this? I will have to upgrade it to 2.0, I guess, and have it unlocked on 2.0. Will I still be able to develop on it with the official SDK even if it has never been activated?
Thanks.

The main issue is that you need to be a legit AT&T customer and have activated your phone with AT&T prior to installing the latest beta.

You could run an Unlocked Baseband with a different carrier, but unless you know how to hacktivate the latest beta, it will do you no good because the moment you switch SIMs it will go into lockdown.

You can fiddle with the iPhone without a SIM, but regardless to be able to do so you will need to have it activated first.

My suggestion is that as soon as you get the 1st gen iPhone, you activate it through iTunes with a factory default 1.1.4.
Once you get the new 3G just commit again to the two year agreement.
Note: the commitment should only be from the day you sign. Your previous commitment should be superimposed by the one signed on the date of your 3G purchase.

By the time you get your 3G, 2.0 should be activated by the hacking community so you shouldn't have any problem keeping your old device for your development under the latest release.