Product: Mobile > Cell Phones > AT&T Cell Phones > GT-I9020A
Yes. Paid apps are tied to the Google account used to purchase them. As long as you continue to use the same Google account on the Nexus S you will always be able to re-download paid apps.
Note: As of this writing, the Android Terms of Service (TOS) allow for an unlimited amount of re-downloads of paid apps using corresponding Google account. The Market TOS is entirely set by Google and could change at anytime without prior notice. As always, carefully read the TOS before agreement.