[tvon]

You can accomplish something similar with listchars and list:

  set list
  set listchars=tab:»·,trail:·

It will not give you a file-wide warning but it will make tabs stand out (along with trailing whitespace).

Not much help if you actually want tabs, granted.

[reaperhulk]

Oddly enough I added listchars data to my vimrc today without even thinking it would be helpful for detecting mixed. This ought to be good enough for now, thanks!