We're ex-Splunk, and can tell you it's a pretty expensive solution if you end up buying it. The free version of Splunk is nice, but you still have to provision a server to run it on.
We are planning providing filtering to Loggly with Jordan's Logstash project: http://code.google.com/p/logstash/ If anyone is interested in helping we'd love to have you.
For folk pushing 1G of logs an hour, it'd be nice to have logic built into a local syslog proxy that filtered/dedup'd the log in some intelligent way.