On average, monsters do around level + 8 damage now, with brutes doing a little more and limited attacks doing a little more again. This damage increase really comes into play at paragon and epic tier, so you can leave heroic monsters alone.
There are two options for older monsters:
A) Use the new damage chart and change expressions as you like.
B) Double the static damage normal creatures do and triple the static damage that a brute does.
EG: A level 12 creature that does 2d6+3 damage, now does 2d6+6 damage.
A level 12 brute that does 2d6+3 damage, now does 2d6+9 damage.
The double/triple method is the easiest one in practice and actually comes off pretty well for monster creatures. For a more swingy effect, you can also add 1 or 2 dice onto the attack. The above example would be 3d6+3 and 4d6+3 for a brute.
Hum...I'm trying to grasp the impact/scope of this errata. Does this change the damage of all monsters & traps in Monster Manual 1 & 2, plus all adventures and other books before July 2010?
Is the online compendium updated to reflect changes to damage of older monsters?
No and no, though many of the older monsters _should_ have their damage changed.
Expect to see some higher damage options in newer books, though. Monster Vault will soon be out, and I highly suspect it'll have some much better versions of a number of monsters.