Does a conference poster count as a publication for PhD application? I'm preparing a CV and required documents for PhD applications and I've been asked to provide references to publications. I've been wondering if it's appropriate and a good idea to put my student poster there?

The field is Computer Science and the poster has been presented during one of the top conferences. Posters were not part of proceedings and are not officially available (only online abstracts).
It's definitely a good idea to include such a poster in your CV or mention it elsewhere in your application (e.g. discuss it and the work in the personal statement). However, I would refrain from listing anything under "publications" unless it has been peer reviewed and published in a reputable journal.

I think in this particular case, the application is asking for references to papers rather than posters, but it may be worthwhile emailing the admissions people at the department to ask where you should include the poster.