Tim,
You are probably thinking of the viewing area on the top of Terminal 2; the much more extensive 'roof gardens' on the Queen's Building (which is located between Terminals 1 & 2) were closed to spectators a long time ago.
From all reports it seems very unlikely that this will ever re-open. Even before the supposed security threats, it was said to be likely to close as it did not offer disabled access and could not be easily modified to do so. Now, as you may have heard, there are plans once Terminal 5 is open to demolish T1, T2 and the QB and develop 'Heathrow East', a brand new terminal to the same standard as T5.
The best one can really hope for at LHR is continued use of the visitors' centre car park, and the 'unofficial' vantage points around the perimeter which have been discussed here before. Try not to let it spoil your enthusiasm for aviation!
Rhys.