Yes, an original iPhone should still be upgradable to firmware 2.0 when it's released. You'll need to use one of the jailbreak/unlock/hacktivate programs though.
If you aren't using an original iPhone as a cell phone on AT&T's service, then installing any firmware from Apple will leave you with a picture of the planet earth and a request to activate the phone with AT&T and iTunes... which leads you down the path to signing up for service.