My problem is exactly the same as the original poster's problem.
If you refer to the thread the problem isn't solved, the poster still has to manually edit the markup generated after using the designer or dropping the control on the form, I wouldn't think having to manually edit the markup is a correct solution.
I've traced the problem to adding the infragistics control tagprefixes to the web.config so they are available in all pages without having to use the register syntax at the top of every page. If I remove the tagprexes from the web.config and either manully put the register syntax at the top of the web form, or have the infragistics designer put the register syntax on the webform for me the control will serialize the markup correctly.
However I don't think this is a good solution.
We do not like to register controls on each page in the solution, as there are hundreds of pages, and due to the desinger being very slow it is generally quicker to work with markup directly, however manually writing markup for grid controls is not fun so using the desinger is preferred.
I'm not certain when this problem started happening, as we have been using infragistics controls since 2004.x versions, but recently started working on new pages using grids with the 2008.2 version and this has started occuring again.
I think I will log a support request and hopefully this can be solved in a future release.