CM: it's instantaneous IAS as reported by mode S from that particular radar sweep, not the target IAS. This is as opposed to selected flight level which is the SFL from your MCP.
http://www.levelbust.com/articles/mode_s.htm has some pertinent info.
@.4 if you catch the lack of speed compliance early enough, sometimes a quick 'xxx123 report your speed' is enough. Responses (when supposed to be at 180 and clearly going slower) have varied from 'xxx123 one-eight- er... one seven, oh we're one sixty' to the blatant 'xxx123 we're at one-eighty, and we have a problem with one of our airspeed indicators'.
<Pause> . 'xxx123....Roger'.
As stated by others, different speed requirements will be accommodated, however we need to know about it first. Otherwise, the tight margins can be eroded very quickly.
Cheers W.E.