(well, it's only Off Topic for uk.railway, very much On Topic for
uk.transport.london, but as I'm x-posting this...)

http://news.bbc.co.uk/1/hi/business/7342263.stm

A few choice quotes...

"British Airways has delayed moving most of its long-haul flights to
Heathrow's new Terminal 5 until June.
[...]
No date in June has yet been given for the move.

The delay in moving will have a knock-on effect on other airlines who
had been due to take over Terminal 4 once BA had left.
[...]
Sixty arrivals and 60 departures per day were due to be moved."