Many users are reporting problems with wireless networking in Leopard. I’ve seen a slightly different symptom on my MacBook Pro. Instead of progressively slowing down, I find that my Airport connection suddenly dies. A few times I’ve seen an alert saying “the network appears to have been compromised and will be disabled”. Other times it would simply stop responding without any message.
Although I was able to get Time Machine to recognize my external drive when connected to the Airport Extreme base station, it was nearly useless. That base station would randomly hang or I would lose the connection. I finally ended up connecting that drive to my iMac and sharing it.
For the last day or so, I was completely unable to connect to my Linksys WRT54G’s 802.11g network. Even removing the entries for that network from my keychain didn’t fix it. Renaming the network finally fixed that problem.