After Yesterday (30th October 2008) Ubuntu 8.10 release a regression was discovered in Linux kernel version 2.6.27 packages.
Details about Linux kernel regression
Version 2.6.27 of the Linux kernel changed the order of options in TCP headers. While this change was RFC-compliant, it was found that some old routers and consumer DSL modems would not route traffic for these systems when TCP timestamps were enabled. As a workaround, TCP timestamps were disabled via sysctl.
The problem can be corrected by upgrading your system to the following package versions:
sudo aptitude update
sudo aptitude dist-upgrade
After a standard system upgrade you need to reboot your computer to effect the necessary changes.