This is apparently related to an issue in Puppet --

http://projects.puppetlabs.com/issues/show/3949

Began getting the following for all reports:

Processing ReportsController#create to yml (for 127.0.0.1 at 2010-06-23 18:19:56) [POST]
Parameters: {"format"=>"yml", "action"=>"create", "controller"=>"reports", "report"=>"[FILTERED]"}
processing report for <puppet_client>
Failed to process report for <puppet_client> due to:undefined method `name' for "RRDtool":String
Completed in 31ms (View: 0, DB: 14) | 500 Internal Server Error [http://&lt;puppet_server&gt;/reports/create?format=yml]

Apparently this is due to RRDTool libraries not being loaded -- adding "require 'RRDTool'" in Foreman's environment.rb clears the problem up.

The fix was suggested by Volcane, it's not mine.