A number of people have had problems connecting to repositories caused by bad DNS behaviour by certain routers. For some unknown reason Linksys DSL-G604T and DSL-G624T routers often return a bogus IP address (188.8.131.52). Strangely, they only do this when queried by certain applications - apt-get in this case - but not when queried by Firefox. You can view websites without any problem, but attempts to install/update software using apt-get or synaptic fail.
The workaround is to bypass the router and configure the connection DNS server settings manually.
Here's how to do that in Xandros Easy Mode