Good grief! This thread's right up my street, and I haven't noticed it until now!
Lorna, it depends on which Snetterton you mean. An unscientific guess - and a quick peek at a track map - suggests to me that the Norwich Straight, which went from just after Sear Corner to a hairpin and then back to The Esses (imagine a triangle drawn at the top end of the current circuit), was longer than any of the current straights.
In terms of other world circuits with long straights, I beleive the longest in F1 is at Circuit de Catalunya, (it's the longest in Moto GP for certain), while the longest in CART competition is at Mexico City. In both cases these are the start/finish straights.
One other circuit with an extremely long straight was the Grenzlandring in Germany. This 5.597 mile oval set on ordinary country roads is basically egg shaped. Given the total length of the circuit, I'd estimate that one of the straights, the Rheydter Gerade, must be at least 2 miles long.
For a track map and history, look in the features section of:
http://www.etracks.freeserve.co.uk/