I didn't read the other responses so if I'm repeating anything I apologize!
If you are getting the infant seat (the bucket type seat) you should buy two bases-one for each car that the baby will be in the most. Then the other cars can just use the seat belt-infant seats have hooks that you can slide the seat belt into over the seat to hold it in place without the base (http://www.youtube.com/watch?v=d9pi6x90lLU)
I has two bases for each of my first two children but only one with our third who outgrew the infant seat way to fast to bother buying another base in the long run-I had big babies who moved into convertable seats very fast, over 9lbs each at birth. . .
All in all I personally won't buy another infant seat, I will just go with a convertable seat made for smaller infants and not deal with lugging around the 'bucket' everywhere I go! Of course this may not be the option for you since you would have to unlatch the car seat and leave it with the day care or family member who will be watching your LO, though with the new latch system if all your vehicles were manufactured after 2003 they should all have it which would make moving the car seat around a little easier. . .
Whatever you choose-Congratulations and good luck :)