RuleChainingBehavior.UpdateOnly has been supported in WF Rules since .NET Framework 3.0. It allows you to explicitly participate in the forward chaining process.
Rule attributes are RuleRead, RuleWrite, RuleInvoke which can be placed on methods that potentially modify or read variables being used in rules.
To learn more about this, please go to http://msdn2.microsoft.com/en-us/library/aa480193.aspx#introwork_topic5.
We are trying to find more about the usage of these features - if you are using it in your WF Rules apps, please email me (contact me through my blog) as I am interested in learning about your scenario. We are considering changing the design here and want to determine the impact and understand what kind of scenarios it is being used for today.