Rather a long title, but I couldn't think of anything shorter. Anyway, it's a topic that I would have preferred was better publicised.
SQL 2008 R2 Business Intelligence Development Studio (BIDS) will only recognise extension components (such as SSIS Data Flow Components) that are built against .Net Framework 3.5. Neither earlier nor later versions will work, and I've yet to find any useful messages to tell you why.
So far I've only done enough testing to find combinations that definitely work, as summarised in the following table.
|BIDS Version||.Net Framework version of component|
|SQL 2008 R2||3.5|