Nope, just a standard Bukkit install. It was reporting online for at least a month, then it suddenly started reporting offline. The only thing I can think of is the fact that we're still on 1.6 and maybe the pinging service is looking for 1.7 servers?
Another 24 hours has passed with several pings, all reading offline, but the server has been running just fine with 10-15 players online at a time. Any word on this issue?