Office 2008 for the Mac will be released for manufacture in December of this year for a US release of mid-January 2008 (just in time for Macworld 2008).
Clearly updating a large suite like Office is always a major undertaking, and focusing on getting it right is great, however, January 2008 is 5 months away from now. I asked if there would, by any chance, a public beta in the coming months. Sadly, there will not be one. It would seem if you want to get your fix of Office 2008 you'll have to check out this gallery of screenshots Microsoft provided to us during Macworld 2007.