Authoring Tools Not Working
The procedure entry point ETWMouseDown could not be located in the dynamic link library tier0.dll."
I get this too. Windows 7 x64 bit? / Comodo internet security?
Well, after having used the 'verify integrity of game cache' option, at least Portal 2 is working again. With a slightly strange behaviour, though. The portalgun potato shows up in my custom map and the fog doesn't seem to work anymore (I already checked the video settings).
The authoring tools window is still blank ;(
(Win7/64-bit)
I was able to run the "C:Program Files (x86)SteamSteamAppscommonportal 2binhammer.exe" -nop4 command with no problem.
Made my first map today, it really sucks compared to the one's I've been downloading.
Sure if you didn't use any models in it. 
There are some changes in the game now I don't know if they were meant to be like this or just some wierd side-effects. (Like the fog as Neco said, or you that you cannot use the "ent_fire changepainttype" in game anymore normally (except for adhesion gel) because it just turns the paint gray and it won't work anymore. They removed the color of the stick paint too (the splash one, the blob texture can still be used).)
Im getting the same "hammer.exe.entry point not found" when i try to load the authoring tools directly from the start menu, and when i load it from steam i just get the blank box with the words authoring tools at the top. I think my problem is that there are two dodgy files in portal 2 and the tools because i used the cache verification tool on both and both had two missing files.
all of our Author tools doesnt work 
Valve do something! I can open portal and i can open the author tool, but the author tool remains blank, with no buttons to click on
Yeah, mine is getting the 'entry point' error now also, even with my fix.
It started after I did the 'validate files' thing
They seem to have totally screwed this up, and no word from them on it
Everytime I do validate now, it says there are files that need updating, even though I download them every time...
Does the SDK? -.^
The funny thing is that when I go through the 'verify integrity of game cache' procedure at least Portal 2 works but when I do the same for the Authoring Tools both stop working and I have to reuse the above mentioned option.