Well, my W H Smiths Wall Planner says it is today....
The table on scienceworld says it uses a mathematical formula which could be an hour out, and seeing as four different sources I have found say that the solstice is on 20th June 2008 23:58 UTC then I can see where the confusion is....
Certainly, the event organised by English Heritage for the Summer Solstice is happening today (Friday 20th June) and the National Maritime Museum also indicates today as being the longest day.
Why not just celebrate both days just in case....