"cfman" wrote...
If the front GUI is in Excel, and the heavy-duty backend
computational engine is in Matlab, what is the best way to
handle this interfacing problem?

Another headache problem is: for the users who only use Excel
and who has no desire to purchase a Matlab licence and work
with Matlab, what's the best way to package that Matlab
computational engine along with the Excel?

That's a MatLab question, not an Excel one. Why not start by doing a
wee bit of research on the MathWorks web site. I just tried that, and
I came away with the following likely links.

http://www.mathworks.com/products/compiler/

and

http://www.mathworks.com/products/excellink/

If the MatLab compiler can create EXEs, there are crude ways to tie
Excel together with arbitrary EXEs.