I'm curious about this question as well, because the upcoming "Essentials" line for D&D 4E doesn't remotely look as if it's trimming stuff down to make it a more compact rules reference. Sure, it's intended to make it easier to pick it up as a game for people who've never hard of RPGs before - which mandates extra verbiage, as per Umbran's post earlier - but providing a simplified rules reference it probably won't.
I'm saying this for a couple of reasons. Our current thread contains some very good observations on what a
compact rules reference needs. Lots of white space, e.g., to make quick absorption of info easier. Great layout. Large font. Incidentally, the D&D 3.5 Rules Compendium had all that (e.g. page size of
10.9 x 8.5 inches). The 4E Rules Compendium, otoh, seems to have none of it. For one, it's a bloated book with 250+ pages. Too long. For another, the size of the pages are reduced -
9 x 6 inches. Which means that information that could be grouped on two facing A4 pages would now need to go on three smaller pages, adding the page flipping. Also, a reduction of font size (seriously, keep the current font size and double columns.... you can't do that on the reduced page size).
That said, you could SO fit 4E in a very slim rules reference. Mind you, the idea here would be to have a Core Rulebook and then to have all classes, races, feats, and gear in separate sources (like... stashes of cards!). Here's my break-down for this, leafing through PHB 1 chapter by chapter.
1. Chapter 1-2: If we go for a compact rules reference, "How to Play" and "Character Creation" are mostly not part of the Core Rules, and to the extent that they are (which in part they undeniable
are) can be compressed into something smaller. E.g., just looking at chapter 2, I'd kick out all that pointless verbiage on what stuff like the tiers actually mean and go for the cold number tables. Ditto for 'roleplaying' and 'deities'. That leaves us with pp. 14-18 (left half of 18 only), 25-28, table on p.29, pp.30-31.
= 11 pages
2. Races - If we follow the aforementioned approach, I think one could condense the strict rules bit a lot. Let's say
4 pages, and that's generous.
3. Chapter 4. PP. 52-59 are essential, rest goes on power cards.
9 pages
4. Chapter 5. All of it, pp. 178-189.
12 pages
5. Chapter 6, feats. Strictly, you need all of it but on my envisaged version I'd either get rid of feats as part of the Core Rules OR (if players feel strongly about them) have cards for them. Partly, some feats will have to made into power cards anyway (see 4.), e.g. Channel Divinity.
0 pages, but that's a cop out on my part.
6. Equipment aka the info 'how much damage does a longsword do'. I think like powers this info needs to go onto cards. Even then, you'll need at least 3 pages explaining the terminology that goes onto that card. The "[W]" which no one found when first leafing through the PHB would need go get explained up front, as would the basic terms on the magic item cards (pp224-226).
5 pages
7. Chapter 8 - "Exploration", "Rest and Recovery" (260-263; and the short paragraph on "Milestones" on p.259).
4 pages
8. Chapter 9 - Combat. All of it. It's
31 pages.
9. Chapter 10 - Rituals.
1 page of explanation of "How to Read a Ritual" etc. Remainder of the chapter goes on a separate stash of cards please.
So in total that's
77 pages. That is the type of book I think 4E players could do exceedingly well with. Fully updated with errata, add in 5 pages for a complete
glossary with all the keywords (basically the final entry in PHB 3). Instead we get
this, "a quick and handy rules reference" which clocks in at 320 pages of reduced page size, and will fall apart after frequent page flipping thanks to
softcover binding. It's dead cheap, which I'm grateful for, but I'd rather shell out $30 and have a solid rules reference that comes in handy for many years to come, lies flat on the table so I can hold cards&snacks and roll dice while I'm reading it
and allows me to gather the information I need at the table in a heartbeat.
I get your point, but on the books issue, we're talking a difference of only five or six years, really. For TV, it's gonna get ridiculous when a "half-hour" show is actually 10 minutes long, with 20 minutes of commercials - and that's maybe 10-15 years away at this point. 
