This is a big deal. While Cisco is the number one router and gateway vendor, Juniper is not small by any means – and with the properly crafted traffic, a Juniper gateway can be crashed. This then affects any traffic that will use that gateway, no matter where it is going.
Releases 3 through 10 of Junos are vulnerable; there is no way to fix this problem with the firewall or other work arounds. Releases that were released later than January 8, 2009, are not vulnerable.
Over at the Praetorian Prefect blog, there is a detailed description of the vulnerability.