I know the recommended cpu is 2 ghz or higher but this was written during the P4 days.  Would a newer architect core such as the conroe 1.8 ghz celeron be good enough for an unraid server?

Yes, a conroe-L is perfect. I clock my 2.66 core 2 duo down to 1.6 ghz and it works fine.

I also have unRAID running on a 1GHZ celeron M without issues.

It's more a factor of bus speed then raw CPU power.

I'm using a Celeron 430 @ 1.8 GHz. Works just fine. As WeeboTech mentioned, you'll be limited by bus speed and hard drive speed before you hit the processor ceiling. Haven't underclocked/undervolted yet, but I think I'll do that when GB-PVR finishes recording (recordings go straight to unRAID server).

I also have a Celeron 430 @ 1.8 GHz - been monitoring CPU usage when copying files to my unraid system. I cant get it to use more than 30% CPU times so this CPU is more than enough.