Is the content caching or not?

[pluggyboy]

test result: http://www.webpagetest.org/result/120509_2R_49HKD/

I'm getting an 'A' for caching static content, but then the detailed view of the second pass is showing that loads of the content hasn't cached: http://www.webpagetest.org/result/120509...ls/cached/

is this down to the fact that there's a version applied at the end of the file name (ie http://d1yvubuvg1uugl.cloudfront.net/wp-...20120110)?

The version number doesn't change from first to second pass, so surely that shouldn't make a difference?

Finally... any idea what causes these version numbers to be applied at a server level and how they can be turned off?

Any help much appreciated, thanks in advance.