Idling is the ability to apparently be present in chatrooms non-stop all day and night, and to read whatever you may find interesting of the discussion at whatever time suits you, and respond in a time-delayed fashion.
This is a great culture that until now has only been supported by IRC technology. Other chat systems allow for entering chatrooms, too, but frequently the user interfaces of client software aren't optimized to support this kind of productive idling. Ergo, this only happens on IRC.
And on PSYC of course. Why? Well, one reason is because many of us use IRC clients to start with, the other reason is because the /subscribe command and the whole subscription model is aimed at making chatrooms and whatever other many-to-many distribution channel more persistent.
For instance newsfeeds do not even appear in form of chatrooms to an IRC client, so you are effectively idling on all those newsfeeds, too, only you don't need to skip over them in your client.
In fact psyced's telnet interface allows for idling on even more channels than is popular on IRC, and PSYC clients SHOULD do so, too. This works by having a more flexible approach to windowing - not statically a window for each channel - or no windowing at all, as in the case of telnet. I am idling on some 30 channels right now, even if I am actively speaking on only four or five each day. That is a very productive use for event-oriented subscription technology.
Some links on the topic of being idle on IRC:
English Wikipedia is still lacking an article on the topic. Incredible! ;) (Wikipedia:Idling is about motors!)
Some Internet games even make it a requirement to idle on their channels to be allowed to play.