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I'm not sure it is a big deal if you expose both to Google. There is a setting in webmaster tools that lets you explicitly set which you prefer: example.com or www.example.com

http://www.google.com/webmasters/

Although, the preferred way is probably to use a redirect.

And a canonical URL in your header.

Just redirect permanently from one to the other.

I believe internet explorer combines the www. and non www. cookies. I remember sending an email explain site who's shopping cart would "mysteriously" empty when it switched to https://www.site.com if you started on site.com. their answer was basically "no, we're not broke." It was kind of disappointing.

it looks like they did fix the problem though: site.com 301's to www and they now set cookies on .site.com