The Day Of The Doctor will celebrate 50 years of Doctor Who, and it appears that the classic series actors will get their moment to shine. Update: But sadly, not on the ABC.
The Fiveish Doctors is currently on the schedules of the BBC, accessed via the red button and set to air following The Day Of The Doctor. It’s described as follows:

Miss the classic Doctors? They’re back in this celebratory special written and directed by Peter Davison.

There also appears to be an associated Twitter account @TheFishDr, which has provided a few cryptic updates.

POLICE INCIDENT LOG – April 18th 2013. Certain items reported missing from Doctor Who experience, Cardiff. #FishDr #FDR

‘Get Sylvester, I’ve had a moment of inspiration!’ – Peter Jackson, April 2013.

‘We’re proud to announce that three of our members have been booked for April 2013’-The Amalgamated Union of Dalek Operators. Cardiff branch

I’ve eaten possums anus on live television, it couldn’t be worse than that’ – Colin Baker. #FDR #FishDr

I’ve put a query through to the ABC to see if they’ll pick up the short special — my inner cynic doubts it, but it never hurts to verify.
Update: The ABC confirms (sadly) that it’s not getting this particular extra bit of 50th Anniversary fun.