It is way too early to be confident there is an update happening right now, but there are some early warning signs.
Pete Hall at WebmasterWorld said:
What on earth has gone down in the UK today?
I've seen a site drop a huge amount. Penguin?
Richard Merry commented here saying:
I'm in the UK and on the 27th July 2012 many of the keywords my site ranked at the top, moved down to over 100 places and a few of my clients sites signicantly moved down as well.
Again, these are just early warning signs.
Forum discussion at WebmasterWorld.