David Gainer was kind enough to answer the mulit thread .xll question

Excel 97 does not support multi-threading. I can see what I wrote was confusing. What I meant was that if you had some XLLs that you have been using for some versions that you want to participate in mult-threaded calc, you can recompile them after adding the flag and they will work – you don’t need to update anything to use the Excel12 API. Is that clearer?

If you’ve not seen Dave’s blog check it out, Dave keeps it updated and replys to many of the comments, very good work!