As of 2010-08-18:

Firefox:
* requires that the type match the Content-Type

Opera:
* drops connection if content-type doesn't match a recognised type
* sniffs for recognised types
* recognises application/octet-stream

Chrome:
* almost ignores Content-Type, always sniffs
* except... for text/html and text/plain, it starts later than t=0 and doesn't let you seek (!)

Safari:
* almost ignores Content-Type, always sniffs
* except... if the format isn't recognised, but the type was text/plain, it reads the whole file then renders it one line at a time

IE:
* ignores Content-Type, always sniffs