FYI, this came across the inbox from a nonprofit I work with:

9/14/09
Constant Contact releases InfoTransfer for Salesforce Plug-In – Good news for our customers who use Salesforce.com’s customer relationship management service! We now offer a plug-in that makes it a breeze to upload and synchronize contacts in Salesforce.com with your Constant Contact account. You can rest assured, knowing your contacts are up to date, and comply with CAN-SPAM unsubscribe guidelines. Log into your Constant Contact account, click on the “Contacts” tab, and under “My Contacts” you’ll see the “Import Tools” link.
After a call to their support line for more info:

• opt-outs synchronized back to SF
• add leads and contacts in SF to a list in Constant Contact
• does not yet work with SF Campaigns or Reports (“they’re working on it”)
• email updates in CC don’t yet propogate back to SF (“they’re working on it”)

So that’s the info.  Not sure if this sways anyone off of Vertical Response, but it will be nice to have another mass-email option, especially for orgs that are already using CC.