Car speakers in the house?

Car audio speakers (if they must be utilized) would be best suited to low-tech rear-channel in a surround system, or perhaps as front or side effects channels in a 7.1 or 9.1 system.  

As has been stated above, the center channel is very important in a surround-sound system.  In fact I maintain it is the MOST important speaker in a home theatre system.  You need to be careful when buying or building to voice-match the center channel as closely as possible to the stereo mains or you can get some strange effects when the sound shifts from one to the other.  If you plan to DIY main and center channel speakers, I suggest you should use the exact same mid-bass or mid-range drivers and tweeters, and design the crossovers so they all match.  Don't use car speakers for any of these channels.  Amplifier selection is of supreme importance as well... but let's not turn this into a home theatre thread.