Can I use a new 2011-issued EIN for 2010 taxes?

I started a new side business in early 2010 I had a couple small clients (that were also friends) that paid over $600 and gave them my SSN when I filled out my W-9s. That was ok because I knew and trusted them. I started doing work for a larger client last summer who just sent me a W-9 in early January to complete. In anticipation of this, I obtain an EIN (also early in January 2011) and want to provide this company with my new EIN instead of my SSN. Is it a problem to use my new 2011-issued EIN for income that I earned in 2010?