What we 'actually' know:

Operating System : Android 2.1
Device Name: Nexus One
Manufacturer Branding: Google
Hardware Manufacturer: HTC
Carrier Unlock: Yes
Network: GSM
Chipset: Snapdragon Processor
Launch Date: January/February 2010
Display: 3.7-inch display
High-resolution Capacitive multitouch OLED screen
Keyboard: Onscreen keyboard only
Hardware Buttons: HTC scrol ball
Size: Thinner than the iPhone 3GS
Other: 2 mics, one in the front for talking and one at the back to reduce background noise

Engaget has the best review thus far (Key Findings):

• By all appearances, the company will have a new phone portal where buyers can pick between an unsubsidized, unlocked Nexus One for $529.99, or sign up for a two-year agreement with T-Mobile and purchase the phone for $179.99
• Google Maps with no pinch-to-zoom
• 5 megapixel lens and flash took sharp (iPhone 3 megapixel, no flash)

Other items of note based on buzz:

• Open system
• Higher Resolution 800 x 480
• Thinner than iPhone
• 1Ghz Processor, 512MB RAM (Almost 2x more than the iPhone GS)
• Video recording 720×480 pixels at 20fps (iPhone is 30fps)
• Same charger for the Nexus One works with Bluetooth headsets
• Alphabetic list of all installed apps
• Does not Sync your Calendar with Exchange
• Replaceable battery
• 10k apps vs. "one meelion"

Guy Kawaski says:

"The Nexus One is a serious challenge to the iPhone—particularly because this is version 1 of Nexus One while the iPhone has been out for years. It is the phone that Palm should have created, but that’s another story.

Right now I cannot use a Nexus One as my primary phone because it cannot sync my calendar (something my teenage son doesn’t care about), I use it all the time (without a SIM card, just as a Wi-Fi device) while my iPhone charges around the house. The day that calendar syncing works will be very interesting."