Verizon was adjusting the Maximum Transmission Unit of all
packets on the fly on their routers - and they stopped doing that recently.
Some sites, Microsoft Update in specific, demand that the MTU be between 1400 and 1536 or they simply won't interact with a browser. Any browser. At all.
Packets that have a frame size larger or smaller than that are ignored or otherwise mangled beyond recognition by any browser I know of.
So, if you are like me and have several locations sharing Verizon DSL
circuits between lots of Windows users via an iptables-based firewall/router, Microsoft Update, mail.yahoo.com, and a bunch of other sites suddenly became inaccessible on February 9, 2009 for no apparent reason.
The fix is to set a "clamp" on packet size via a variety of iptables
commands, I like to do it in the FORWARD chain like so (all one line):