I am afraid we are not able to reproduce this issue in our local tests. A GAC'ed assembly can only be used with its fully qualified name. Is it possible that the problem has been caused by caching? We did try this scenario with and without placing the new Telerik.Charting in GAC and either way the ambiguous error disappeared as soon as we removed all the references to the old assembly.
Anyway, I am glad you have resolved it and I can assure you that with Q3 2008 this will no more be a problem, as we will IL merge Telerik.Charting into Telerik.Web.UI, thus eliminating such issues.
All the best,
the Telerik team
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