I didn't know the poll would generate so much interest in credit scores. Here's a good piece on credit scoring:
http://www.bankrate.com/brm/news/spe...sp?prodtype=cc

The number of credit cards you have (and how often you switch) plays a part in your score because one of the numbers computed is your average length of credit history. Having accounts for a long time is better than having them a short time and every time you switch you are reducing your average.

But bear in mind that length of credit history is just one of many factors that's used to get a credit score.