nimrodeo,
You're looking for the stock to be 'in play' which means that a lot of attention is directed towards it suddenly. Usually this is coupled with an event which puts a lot of people on the wrong side and gets others without a position to come in and take advantage of it.
We're looking for rel. high volume, high trade count (*a lot* of trades going off, not one or two big blocks), price volatility (gaps and such) as well as news (as already mentioned).
RIMM would be a good recent example (Sept 29 2005 and on Friday Oct 7 2005). It gapped down on negative legal news, the volume spiked and trade count was through the roof.
In the end this is discretionary, you will have to define what is 'unusual' - or you can decide to let a mkt scanner do it for you - or as I've mentioned you can pick a strongly trending stock and find 'dummy' intraday entries into the eod trend. Or play it as a swing trade via eod. Or develop your own personal tweak.