People that play on servers may regret updating just yet. Servers that run mods often can't update as quickly as clients (your computer); and as a result players may find themselves unable to log on to a particular server.
The problem is simple enough to fix. Just got to: C:\Users\[your user name]\AppData\Roaming\.minecraft\bin and rename the minecraft.jar to minecraft.jar.bakup
Then place the 1.8.1 mineraft.jar file in the bin folder
If you don't have the 1.8.1 minecraft you can download it here: 1.8.1 minecraft (warning: not my link or file so use at your own risk; I make nor imply any responsibility for the file). Click "skip add" at the top and the download should start.
Extract the minecraft.jar with winrar or 7zip and put it in the bin folder.
If you can't find the AppData folder you'll need to go to: Control Panel/Folder Options/View/ and check show hidden files, folders and drives
Now you should be able to play 1.8.1 again!