What distinguishes between the two states 'idle' and 'in-use'? Is this some sort of system defined criterion or is it dependent on the CPU load defined in BOINC, i.e., 'suspend if CPU exceeds X%' value?

I believe I figured it out. From the Wiki (http://boinc.berkeley.edu/wiki/Preferences)

Use at most X% of memory when computer is in use

Limits the amount of RAM used by BOINC applications when your computer is in use (i.e. when there has been recent mouse or keyboard activity). Setting this to a low value lets you run BOINC all the time without impacting your computer's performance. The Default is 50%. - [emphasise mine]

It would appear then that a WU with a 'working set' size larger than the 'in-use' memory threshold would cause the WU to either swap out - 'keep WU memory resident when suspended' - or outright terminate (and begin/resume another WU if available to process), when either mouse / keyboard activity occurs. If the latter condition arises, I'd assume the status for the terminated / swapped-out WU would be 'waiting for memory'.

Almost. What i.e., 'suspend if CPU exceeds X%' value? means is that if you have something else running that uses over 75% of your CPU, BOINC will suspend itself to get out of the way so the other program can work. Most people have found this annoying and have either set it to a much higher value or to 0 which disables it completely. This was put in because some people complained that BOINC was interfering with their other programs or games.

As to the idle/in use states,this tells BOINC you have moved your mouse and for it to go away and let you work. When your mouse hasn't moved for a set amount of time BOINC will start again. The Use at most X% of memory when computer is in use limits the amount of RAM so that BOINC doesn't hog too much and other things can have what they need.