I'll buy the first three core rulebooks (PHB1, MM1, DMG1) whether 4E is closed or open. My reason is simple: one of my friends will be running a 4E game someday, and I'll want to play. Therefore, I may as well have the basic rules.
However, I've already decided that the only books I'll buy after that are those which have Open Content, under whatever license they care to publish under. Having gone through purchasing all the 3E books and about half of the v3.5 books before hitting Buyer's Burnout, I have decided that I will only pick up 4E books that contain material that a third-party publisher can use. I dabble in third-party publishing as a minor hobby, and so this seems like a good decision point to determine whether I should pick something up or not.
If WOTC does not follow through on their word and publish Open Content, I can almost guarantee you that within a year, someone will publish a 4E-compatible OGL product similar to OSRIC for the sole purpose of allowing people to publish 4E-compatible OGL products of their own. Sure, it might say Talents instead of Powers or something like that, but I'm pretty sure we'll see it.
My Two Coppers, Anyway,
Flynn