I have the same problem with my PS3, I have the same router, but am using neros media home 4.
Homegroup needs to be set up to allow sharing (even if you have no other Windows 7 pcs), also it only seems to work with WEP security,not wpa2.
Made the changes and now my PS3 picks up both the windows & the nero servers.
Not to happy with wep security, but it's the only way around it.