We ran into this problem and started using the HeaderFilterAgent from Codeplex as well.
We are going to be installing Rollup 7 for SP1 this weekend and was going to implement an increase in the quota limit on all 20 of our databases. If this is going to be included in a rollup pretty soon, we may wait on that.
I am waiting to see some reponses back from the MSEXchange team to my comments, and some of the ones above to see what they suggest we do if we currently are getting errors about not being able to add X-Header information in our event logs.
Do you suggest we still use the HeaderFilterAgent until the Rollup comes out, and will it be needed after this rollup? If we increase the quota now, can revert back to the default after the rollup, or will that value not even really be needed anymore?
Other people that have blogged on the issue:
http://www.eggheadcafe.com/conversation.aspx?messageid=33273938&threadid=33273938
http://anewmessagehasarrived.blogspot.com/2008/10/named-properties-bloat.html
http://www.exchangelets.com/2008/01/mapiexceptionnamedpropsquotaexceeded.html
I was really hoping a solution for this would come out, but I just have some more questions, so I hope the team has time to respond.