Before I installed the DIR-825, I had a DGL-4500 with 802.11n only enabled and a DAP-1555 with 802.11b/g enabled. I was able to wirelessly sync my Zune HD with this previous setup.
Now, I have my DIR-825 broadcasting the same SSID for both radios. My laptop is configured to connect to the 5Ghz radio, as it has a Centrino 2 chipset supporting 802.11n. My Zune HD only support 802.11b/g, and can connect to the 2.4Ghz radio just fine (I can browse the marketplace and access the Internet from it's browser.) It appears as if the broadcast traffic my Zune HD generates to connect to the Zune Software running on my laptop, is never forwarded to my laptop. Thus my Zune HD does not connect to my laptop with this setup.
Even If I connect my laptop to the 2.4Ghz SSID, my Zune HD cannot sync with my laptop.
No, I do not have Wireless Partitioning or the Guest Zone features enabled.