There should not be issues with this. Silverlight 2 is a superset of Silverlight 1.0 and so the runtimes should be fully compatible. In fact, if the dev has Silverlight 2 installed and the Silverlight 2 Tools for VS, they can create a Silverlight Script Web project (instead of Silverlight Application; Script web is under New Website, not New Project) and this should provide a Silverlight 1.0 template to use.
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SDET, Microsoft Web Developer team