I've been getting intermittent 400 errors as well, both on the simulator and on hardware.
Been having a hard-to-repro issue where my server is functioning as well as normal, sends back a 200 response code, and I get a 400 error back from my networking thread.
Looking a little deeper I see this in the MDS logs:
GET /go/mobile/mds/http/mappings2prop.xsl HTTP/1.0
HTTP/1.0 400 Bad Request
XSL stylesheet not found http://www.blackberry.net/go/mobile/mds/http/mappings2prop.xsl: Resource could not be found
It appears as though MDS is requesting this XSL file and, failing to get it, is failing my HTTP request even though everything else is ostensibly working...?
Has anyone diagnosed this issue further? I'm still trying to figure out what all the MDS log messages mean... Can't tell yet if this is truly a catastrophic event or if I should just throw out 400 response codes and carry on...