I had a whole post to write about Paul de Filippo's Slipstream Canon, first presented at the ReaderCon panel that I witnessed. But Paul Kincaid already wrote it for me.
Shorter Paul Kincaid: The definition of slipstream is apparently anything kind of wierd that we like. As Kincaid said, everything on this list (at least everything I've read) is an excellent work of fiction and highly worth reading. Fillippo and his fellow panelists have excellent taste in literature.
But in terms of coming up with a definition of a genre, they've just painted the target around wherever the arrows hit. At the panel Graham Sleight went out of his way to say that slipstream fiction is not the same as postmodern fiction (or interstitial, or etc., etc.) But postmodern fiction's glory days were a decade ago, and during that time 90% of this list was claimed by them.
What is the point, really, of coming up with a term for this? Bertrand Russell went out of his way to create a theory of sets that didn't allow for the paradox of "The set of all sets that don't include themselves." (Yes, his theory was later detonated by Godel's incompleteness postulate, but that's a different story). What's the point of coming up with a genre that defines all books that don't fit in a genre?
A genre, let's face it, is a marketing tool. I'm not even talking about the narrow definition of genre (scifi, fantasy, western, mystery, etc.). This is equally true about the broader definition of genre (poetry, fiction, drama, etc.). And having these categories are useful things, up to a point. But when an idea encounters an area where it's no longer useful, well, then...don't use it.