There has been many long discussion on the Mitel devinfo forum on this very topic. The reason that Mitel has stuck with kernel 2.2.19 for so long is two fold:
1. Kernel 2.4 had a very bad beginning in terms of stability. Kernel 2.2.19 was had a proven track record in terms of stability and therefore made better sense in terms of server reliability. What's interesting is other companies that offer gateway solutions similar to SME are still using Kernel 2.2.19 as well (e.g., Mandrakes Single Network Firewall solution). So, others seem to be on the same page as Mitel on this topic.
2. Several of the ipchains solutions that were implemented in SME did not exist for iptables. Therefore, switching SME to kernel 2.4 would result in a lose of gateway functionality.
Recent info from the Mitel folks seems to indicate that these two issue have been, or have nearly been, solved. I'm betting that a version of SME based on kernel 2.4 with a journaling file system isn't far off.
Regards,
Greg Zartman