I recently developed a spatially-aware .NET application that did not use SQL Server 2008 as the backend (this enterprise was still on SS2005, but we needed the spatial support in the application today). While the application worked properly on my laptop, it was a huge failboat when deployed to the server environment.
I had previously posted that you can get the Microsoft.SqlServer.Types library from MS Downloads, but it turns out that this alone is not sufficient to allow your application to run. You also need to ensure that the SQL Server 2008 Native Client is also installed (regardless of whether you're accessing a SS2008 instance or not). Update! You actually don't... read below.
Both the Types library and the Native Client can be downloaded from the following:
http://www.microsoft.com/downloads/details.aspx?FamilyID=228de03f-3b5a-428a-923f-58a033d316e1&DisplayLang=en
My discovery source: https://connect.microsoft.com/SQLServer/feedback/ViewFeedback.aspx?FeedbackID=355402&wa=wsignin1.0
UPDATE: Per Isaac Kunen (in this blog post's comments as well as offline discussion), the missing component from the Types library is simply an updated version of the C Runtime. The fix of using the Native Client is a hack in this case because its MSI actually installs the updated CRT (which the MSI for the Types library should have done also - it's a goof that MS Downloads hasn't been updated with an updated version of the Types API after the above Connect feedback was answered...
The Microsoft Visual C++ 2008 redistributable by itself can be downloaded from:
http://www.microsoft.com/downloads/details.aspx?FamilyID=A5C84275-3B97-4AB7-A40D-3802B2AF5FC2&displaylang=en