Oh, I forgot about Monster errata - I print out most monsters I use on 4x6 index cards (from the .pdfs when they were available, from the Monster Builder for newer books) so any of the significant monster errata have been penciled in on the relevant cards.
The books are still untouched, and I rarely reference them unless I want to say, run an entire encounter of bullywugs.
I'm curious to hear from anyone who does actually maintain an updates binder. I've found it necessary for wargaming, and before MKII came out I had a bunch of printouts of errata and FAQ answers for Warmachine/Hordes, but haven't really seen much of a use for D&D.
Are there really that many general errata that need to be referenced in play, as opposed to at character generation?
All I can think of off the top of my head are the Stealth changes and new Skill Challenge rules, both of which have been updated in printed products.
Everything else just seems to be a minor, character specific change to keep magic items, powers, and feats balanced.
When we played 3E (pre-Rules Compendium) I needed to have the polymorph errata on hand, but so far nothing so radical has been introduced that I can't just wing it.