rgrgrgrobert,
Very good question.
ACK is actually one of the more self-explanatory acronyms in the Information Technology industry. It simply refers to "acknowledgment".
In your case, your logs are stating that your router is sending an acknowledgment to your wii's IP address to verify that it is present on the network. This will occur with any information exchange between your wii and the rest of your network. ACK is an integral part of the TCP/IP protocol, and is used in any exchange of data, regardless of whether a router logs it or not.
FYI: When posting any questions with your network, the information you want to keep private is anything that can be seen by the public (example: public IP addresses/domain names). Your wii's IP address in this case is most likely private (example: 192.168.0.x, or 10.1.1.x) and therefor inaccessible by the public, hence not a security risk.
Hope this helps! :)
- Joshua
PS: You can read up more on ACK and how it is implemented by the TCP/IP protocol.
http://en.wikipedia.org/wiki/ACK_(TCP)