There has been two updates released for the last two days, with no patch notes, no warning, and it is turning into another Valve ARG.

The next update to TF2 will be the 300th update, so people are speculating that tomorrow could be Meet the Pyro and/or the Mann vs Machine update.

Here's a breakdown of what has happened so far via reddit; http://www.reddit.com/r/tf2/comments/veu75/psa_arg_update_explaining_like_youre_five/

sweat... i actually though there was already a meet the pyro somewhere on youtube