In the past, the ISI Web of Science appeared only to include those meeting abstracts that appeared in an actual print journal. That is, some academic societies will publish the text of the abstracts from their annual meeting in a Supplement to their captive journal.
I've recently noticed that ISI now seems to include presentation listings from societies which do not make their abstracts available in print anymore. Such as the Society for Neuroscience.
Among other things this will be a pain in the ass for looking at the citation report summary stats because they are included in the "items published" per year stats. Personally I'd like to see a default setting where you can exclude those.....now you'd have to go and use the checkbox to exclude the abstracts.
Don't get me wrong, there are reasons to like the inclusion of the meeting presentations. You can do things like look back at how often a person's meeting stuff turns into a paper, perhaps find a half-remembered abstract and thereby remember who to email for details, etc.
But this brings up the question of whether to cite meeting presentations in your publications. (the real ones). Will ISI index them and use them to contribute to your h-index? then heck yes you should cite them. Is this an encouragement to cite the abstract as a way to regain priority for that paper that you got scooped on? To get credit for a project for which you ultimately didn't appear on the author line?
The possibilities are fascinating....