These are the slides for my lecture given at the Workshop on Topological Methods in Combinatorics in Stockholm on June 1st, 2006.
References on the slides (arXiv links are active) are given only for additional results not stated on the slides.
A preprint covering the first three sections of the lecture is available from the arXiv.
A short note covering the last section is being worked on. If you would like a draft of it, please send me an email message.
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