Finally, Channel 5 have confirmed on their FAQ page that they will be showing Season 5:

 

Series 4 of The Mentalist concluded on Friday 1st June. We have acquired the broadcast rights to Series 5 and it is likely that this series will be transmitted later in 2012. Please keep an eye on our website or the TV Guide on the site for updates.

 

Also article about it, published today:

I have just learned that Channel 5 has closed a deal with Warner Bros. International Television Distribution to acquire the exclusive first run UK broadcast rights to the fifth season of The Mentalist. I’m also being told that the show’s fifth season is currently scheduled to premiere on Channel 5 in either late October or early November.
Channel 5′s deal with Warner Bros. to acquire the rights to the fifth season of The Mentalist is expected to be the first in a long list of deals that the broadcaster will be signing in the coming months to secure subsequent seasons of their top performing US imports. My sources tell me that Channel 5 is in talks to pick up subsequent seasons of CSI, CSI: NY and Once Upon A Time. I’m also told that the company will likely pick up the rights to the fourth season of Justified, but the fates of Californication and Parenthood are less certain.
TVWise will bring you the UK Premiere Date for the fifth season of The Mentalist just as soon as it is announced by Channel 5.