There is some helpful information from Ewald Hofman on passing in parameters to the TFS SQL Server Analysis Services OLAP reports.
But, the 2010 schema has changed. You can use the SQL Server Mgmgt Studio to access the metadata and query the tables to seee the new schema and record values.
As an example of the Task Burndown report on the TFS 2010 Project Portal, here's the URL that worked in a browser:
http://{your tfs server}/Reports/Pages/Report.aspx?ItemPath=%2fTfsReports%2fDefaultCollection%2fCcMsd1.0%2fDashboards%2fBurndown&rc%3atoolbar=false&rs%3aCommand=Render&IterationParam=[IterationName].[ParentIterationSK].%26[{the IterationSK value}]
Here's the URL that was accepted in the web part:
/sites/DefaultCollection/CcMsd1.0/_layouts/TfsRedirect.aspx?tf:Type=Report&tf:ReportName=Dashboards%2fBurndown&tf:ShowToolbar=0&Width=381pt&Height= 180pt&tf:Command=Render&tf:Parameters=IterationParam=[IterationName].[ParentIterationSK].%26[17]
To-date though, even though the report processes without error, the passed in parameters don't make any difference/change in the report. So, I gave up on this approach and was somewhat successful with this approach (see my subsequent blog article).