According to RFC2181, a CNAME cannot be at the apex of zone. So this setup is not valid:
example.com IN CNAME lb.default.example.com
lb.default.example.com IN A x.x.x.x
lb.default.example.com IN A y.y.y.y
Is there any way to setup A records for the root of a zone? Like this maybe (depending upon the setup though):
example.com IN A x.x.x.x
example.com IN A y.y.y.y
*.example.com IN CNAME lb.default.example.com
lb.default.example.com IN A x.x.x.x
lb.default.example.com IN A y.y.y.y
PS: PTR records would be nice also.