ACT! 2010, OpenDNS, & peer-to-peer networks don’t mix.

OpenDNS is an incredble service, and I highly recommend using it. Word of caution though. If you are sharing an ACT! 2010 database across a peer-to-peer network, the computer names will attempt to resolve to the OpenDNS servers (on the Net) and your other ACT! clients on the network will be unable to attach to the database. A workaround is to use IP addresses, but it's simpler to remove the OpenDNS server settings and use the default DNS for your upstream ISP.

Otherwise, you'll be looking at the likes of this:

Error: "The database <Database name> could not be accessed. In order to access this database, check your network connection and verify that your database server is available. It may be necessary to disable any firewall software...