Yeah the damage numbers have definitely jumped up and caused the party to take notice. Getting hit for 20-30 damage (at 11th level) is really not all that unusual anymore.
Personally though, I think that there are several solutions for keeping the 3 - 6 encounter/day setup viable. First, as others have said, you can reduce the encounter levels and still threaten the party. For a while there, if the encounter wasn't at least a couple levels above the party's level, there really wasn't any challenge. Now, a level 11 challenge is perfectly challenging, particularly if you use terrain, tactics, etc. that can give the monsters an edge. This also has the advantage of enabling more encounters per level for those groups that track XP. I think this can be fairly important, particular as the party starts heading toward paragon as they learn how to use their characters better.
Second, consider using minions more now. Minions even do more damage as well, but will of course, still only require one hit to bring down. A couple of brutes or an artillery, etc., will still provide enough of a challenge, particularly if they are getting some sort of assistance from the minions (whether its acting as a meat shield, aiding the attacks of others, setting up flanks, or just distracting the attention of the party, etc.).
Next, encourage the party to think about tactics a little more. Before the MM3 update, tactics didn't really make a huge deal (other than maybe focusing fire) as the damage output of the monsters combined with the healing abilities of the PCs, made for pretty easy encounters unless you were at the aforementioned level +3 or 4, etc. Tactics include not only in-combat decisions, but feat/power selection as well. As stated, there's a much bigger incentive to concentrate on defense now for the PCs. Sure, the numbers may seem negligible on paper, but I think they are actually bigger than that in practice.
For instance, while Improved defenses is nice, I think the Great (and especially Superior) Fort/Reflex/Will feats are even better. Let's face it, my dwarf who has nothing but 1/2 bonus to his Reflex, is pretty much always gonna get hit by an attack targetting reflex (even with a +1). But give him another +2 to his Fort (and combine that with a resist to ongoing damage to boot) and suddenly he's saving a lot more damage on him. Just like that same dwarf is probably better off taking Shield the Fallen (which can help all of his teammates from time to time), than say a small damage boosting feat.
Additionally, particularly once the party hits 5th level, they need to realize that they don't simply save their dailies for the big bad boss fight. Even at 5th level, a party of 5 has 10 dailies, meaning if they use two per fight they can still get through 5 fights. Sure, they might not need the daily to win the fight, but using that daily in an easier fight might save the party a few surges. This only increases as the party levels up. By 11th level (where my party is now), they have 15 dailies (mine actually has 18 being a 6th PC party) which means that really, there isn't any fight in which they shouldn't be using at least 1 or 2 dailies.
Of course, none of this stops the fact that monsters absolutely are a lot deadlier now. I think most groups will have to take some time to adjust to this fact. I remember some of my players use to intentionally provoke OA's in order to allow the fighter to punish the monster. They won't be doing that now though as provoking might mean getting tagged for 30 or more damage. Plus, seeing the ranger take 114 points of damage on the first turn of the combat (admittedly it involved an elite using an action point), was just fun.
Edit to add: BTW, my group in their first set of fights at paragon have so far triggered two fights back to back, and seen the ranger take 114 points of damage at once with no fatalities and enough steam to still push on to the 4th encounter in the day. They'll be starting that encounter at the beginning of the next session so we'll see how it goes. Admittedly, being that they are paragon tier, its a bit easier for them to adapt, but I certainly think even a heroic tier group could handle several encounters in a day if they are careful.