i come from shooting a few years on a Canon 1D (before my 1Ds2), which was 4mp. I felt that it was just enough to print decent 8x10s (but only because the sensor had a weak AA filter which gave nice and sharp files!). I think that 6mp is about the threshold for consistently getting decent 8x10 prints. If you don't plan on printing larger than 8x10 very often, or don't usually do alot of cropping on 8x10 prints, then 6mp should be fine
In more practical terms, an 8x10 print at 300dpi requires a 3000x2400px image. 6 megapixels in a 2:3 ratio DSLR is right at about 3000x2000, and when cropped to a 4:5 ratio for 8x10 prints, you end up with 2500x2000 which prints an 8x10 at 250dpi without up-resing. 250dpi is fine for decent 8x10 prints. Probably indistinguishable from a 300dpi print unless you are looking very close at the print.
Now, as you can see that leaves you with very little room (none!) for cropping and leveling, if you don't want to up-res the file to 8x10 or print at a lower DPI.
So, I think your print size should decide your megapixel size. 6mp is the threshold for decent 8x10s, in my opinion. If you want to print 300dpi (without up-sizing) and have the ability to crop, and level your image before printing an 8x10, then you'll want something larger than 6mp. But in practical terms, 6mp is ok for general 8x10 printing.