Is there still a shortage of available inbound connection slots? David Schwartz's answer to this question states the following:

"For some reason that isn't well-understood, the Bitcoin peer-to-peer network is consistently short of available inbound connection slots. This makes it more difficult for newly-started clients to establish a resilient connection to the Bitcoin network."

Is that still a problem? If so, could it be because of some sort of attack (or selfish behaviour) on the Bitcoin network?
It is due to a lot of people not port forwarding from their routers to their Bitcoin machines. Without the forwarding the client can only make outbound connections to those client that have port forwarding enabled. The limited number of such clients is probably due to people not realizing this need and now knowing how to change their router options.