I've developed an SL4 app with various textblocks on the page, in a ViewBox. On my dev machine and home PC, the app looks fine, with text weight and size correct.
On some fresh PC's, with W7 or XP, the Font weight seems to go Bold, and is not fitted into the page correctly. The Viewbox is there to fit the tect into any screen resolution, which works well on most machines, but others seem to distort the text.
I haven't been able to nail it down to any single difference on the machines, but wondered if the lack of .NET 4.0 would have an affect. Is something missing from the SL4 runtime, which is needed for the viewbox to function properly.