Does anyone else find it ironic that the site selling the idea that you need a short (<6 letter) domain has a 12-letter domain name (domainpigeon.com)?
The key is for the domain to be memorable. If you stick two words together, especially ones that normally don't belong together, like domain and pigeon, you make it memorable. If you don't stick two words together and you want something more... web 2.0ish... then you're much better off sticking with something short than something long.
