There is a bug in the report viewer control.
When you install SP1 for VS2010, it adds new assembly versions for the report viewer to the GAC.
The new version contains the bug.
Unfortunately it's not possible to specify, within your config, which version to use as all versions have the same assembly version number. This is done to allow easier upgrade of the report viewer component. But it also means that once you install the SP1 component it's very difficult to roll back.
The SP1 runtime version is: v2.0.50727
Make sure you use an earlier version on your live servers, such as the one referenced in the connect article.
The good new is, if you haven't installed the SP1 viewer into the GAC on your production servers, then you shouldn't experience the problem in your live environment.
There is currently a connect article showing progress with this fault. There is a work-around which involves using GACUtil to remove the SP1 assembly versions, but this could create issues for you at a later date and make VS2010 unstable.http://connect.microsoft.com/VisualStudio/feedback/details/652114/reportviewer-drillthrough-broken-after-vs2010-sp1