I made two sample projects. One under VS 2005 using .Net 2.0 and another under VS 2008 using .Net 3.5. In both of them the spell check is working. First I added the editor on the design surface and run the page.Then I removed it and pasted your code to add it dynamically. I am attaching a movie that I made to see that everything is working. I suggest to open a support ticket and send us a sample project with detailed information how to reproduce the problem.
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