Since Media Center has Style as a default tag now, with "String" as the data type it is not possible to import the full semicolon delimited list into this field. I believe that if you create a custom field ("Style2" for example) with a Data Type of 'List (semicolon delimited)' you should be able to accomplish what you're after.
-JB
I have tried creating a new list field, and copying from the Style field to the new field, but again, the list only contains the first item. If your assertion is correct, and string fields with semicolons will always truncate at the first semicolon, there is a considerable loss of information in my library when importing into MC13. There are other fields - Composer, for instance - that similarly contain multiple entries in a semicolon-delimited list. I suppose this is partly a consequence of the lack of standards in how various fields are represented, but nonetheless, I would hope a tool as generally configurable as MC would find a way to handle this type of situation.
I could, I suppose, use a tool like MP3tag to rename or copy the Style field for every CD I rip - a pretty unfriendly, error prone solution, though.