chksix,
I can assure you it's no myth!
The P-38 "Lightning" was known for its low critical Mach number. I've seen figures as low as M 0.65 quoted. This was due to its thick wing.
Figures for the Spit can be found here:
http://en.wikipedia.org/wiki/Superma...titude_records
That lists M 0.89 as the maximum in a controlled dive, and M 0.94 in a somewhat less-than-controlled dive, which tallies with the figures I remembered from other sources.