I explored all these options: hover, expand, twitter-esque sliding pane etc etc and all have pros and cons:
hover if it is unlikely word count will over run contents of cell. you can see the problem if every cell has to expand on hover and how annoying that would become.
expand seems to work pretty well if the hidden content is not too expansive. this approach also hands control back to the user that on hover does not
new window / sliding div or whatever works if the cell contents are really large, but then you have to ask yourself, why are you putting this kind of information in a table? Having said that, this approach is great for storing extra attributes that are tangentially linked and helpful to the object in the cell and I have used it as a way of allowing users to scan a list of objects, identify the most relevant record and display a div with all attributes associated with that record, an example of this might be a table of holiday notes where the cell lists the description of an activity for the user to identify and on click reveals a window, pane, whatever, of details related to the activity eg price of hire car, lunch details etc etc.
Not a good approach for when the user has to compare columns though.
But the bottom line is, whichever option you go with, test!