OpenVZ is (and has been, for the past few years) a good contributor to the mainline kernel. But in this release we are really doing better than before: 215 patches written by OpenVZ people submitted to the 2.6.24 kernel during the period of its development (i.e. last 3½ months). This is about 2% of all the patches that were merged into 2.6.24.
Most of that patches are for PID namespaces, preliminary support for net namespaces (i.e. network stack virtualization for containers), and various bugfixes.
PID namespace is now almost complete and quite usable, although it's marked as "experimental" for now. For the technical description of the feature, see this lwn.net article.
Net namespace is a work-in-progress, and there are already a lot of patches stacked in Dave Miller's net-2.6.25 tree for future inclusion into the 2.6.25 mainline kernel. The feature is expected to be complete and usable by 2.6.25 kernel release, with IPv6 support coming a bit later.
Jon Corbet of LWN.net also wrote about the 2.6.24 kernel statistics (back when it was still at a RC stage) here. Note that OpenVZ's Pavel Emelyanov is number 5 in "Most active developers" (by changeset) list, with 146 patches contributed.