Despite what I am reading from sources as mighty as former Ashes winning captains Mike Brearley and David Gower who believe England captain Andrew Strauss should have enforced the follow-on, I am in complete disagreement about this. With a lead of only 210, sunny conditions not conducive to swing and seam (it looked flat all day), and at least one bowler with questionable fitness (Flintoff), Strauss made the correct call.
An important point that I feel hasn't been iterated is that run scoring was much, much easier for England with a starting buffer of over 200. From the outset they looked relaxed and inclined to adventurous strokeplay, none better shown than Alistair Cook, normally an unsure figure at the crease early in his innings who pulled freely and even lofted riskily over the off side field. Consequently thanks to a late burst from Matt Prior England scored at 4.36 runs per over for the day and good use was made of the time taken out of the match.
Batting in that manner and scoring at this run rate would have been entirely a different matter had England asked Australia to bat again first, who while under the gun may well have racked up 400 or even 500 (they scored 674/6 in the previous match remember). England could conceivably have been left with a final day run chase of 200-300 or even more, and the pressure would then really have been on. History has shown that sides batting last rarely scale significant size run chases. Instead, with England motoring away Ponting was right back on the defense and England felt under zero pressure. Australia's only chance of getting back in the game was to bat again straight away, and fortunately for England, Strauss did not give them that chance.
If it is overcast tomorrow England may well win with a day or so to spare. If it's sunny expect this to be a real long haul. But I can't remember the last occasion Australia had to bat for this long to save a Test. Not I suspect for a generation...
As a footnote, Strauss will definitely have been influenced by the Trent Bridge Test of 2005, where Michael Vaughan opted to enforce the follow on, only to see the Ashes almost blow up in his face as England collapsed in their subsequent run chase and only just scraped through.