The megapixil rating basically correlates to how big you can print. It doesn't really have anything to do with image quality per se. After the 6 megapixil mark, the image quality is generally good enough for most uses. You really only need more than 6 megapixils if you plan on printing large prints. More megapixils also lets you crop more and still have good sizes for prints. In other words if you screw up your composition in the field you will have more latitude to fix it during post-processing by cropping.
That said, certain professional level endevours do have megapixil requirements. For instance some of the microstock sites might not accept a 6 megapixil image as clients wouldn't have as much flexibility for cropping or print sizes.
Most of the mid-range DSLRs seem to be in the 10-12ish megapixil range, which should serve just about any use you might start out doing. If you want to start out shooting photographs for billboards, you might want to consider a medium format camera instead of a 35mm format. A good 36ish megapixil Hasselblad will only set you back around $30,000-$40,000 US.
