Lets hope this just has to do with some growing pains as Microsoft plans to launch the NXE this Wednesday, November 19th, but it appears that some users may be experiening random disconnects or the inability to connect altogther to Xbox Live.
After several updates, the good Major is still reporting of the issues. Hopefully this doesn't put a damper on your co-op plans, and lets hope they can get it sorted out by the time Left 4 Dead hits this Tuesday.
Update: These issues should be resolved.
Source: Support.xbox.com