SurveyUSA is out with approval rankings for all 50 Governors.  Four out of five with the highest numbers are Republicans (Rell, Hoeven, Rounds, and Huntsmann) as are five out of six with the lowest (Taft, Murkowski, Fletcher, Schwarzennegger, and Blunt). John Lynch is the highest ranked Democrat (#5) and Kathleen Blanco the lowest (#47). 

Taking a quick look at 2006, of the 18 governors currently with approval ratings under 50%, 10 are facing reelection and three are leaving open seats. Here is the breakdown by party:

Republicans  - 4
MD: Ehrlich (48% approval rating)
TX: Perry (47%)
CA: Schwarzennegger (35%)
AK: Murkowski (26%)

Democrats - 6
PA: Rendell (46% approval rating)
WI: Doyle (45%)
OR: Kulongoski (44%)
MI: Granholm (40%)
ME: Baldacci (40%)
IL: Blagojevich (38%)

Open - 3
CO: Republican seat, Owens (48% approval rating)
NY: Republican seat, Pataki (45%)
OH: Republican seat, Taft (18%)

Overall, we're looking at 36 governors races next year with Republicans defending 22 seats and Democrats 14.  In addition to the three open seats mentioned above there are five others:  Arkansas (Huckabee-R), Florida (Bush-R), Iowa (Vilsack-D), Nebraska (Heineman-R), and Nevada (Guinn-R).