Hi Blair

The new SDK sample code is written only in terms of the XLOPER12 structure
which is not supported in earlier versions.  To create XLLs that will work
with 2007 and earlier versions, you have two choices:
1. Use the old SDK.  The resulting XLL is fully supported in Excel 2007 but
will not be able to take advantage of some of the new C API features (access
to bigger grid, multi-threading, Unicode strings).
2. Create your own library or project that detects the running version and
combines XLOPER and XLOPER12 functionality, and uses XLOPER12s only when
running 2007.

If you decide to go down the second path, here's a quick summary of the
differences between the old and new C API that you will need to consider:
1. XLOPER12 added to the new, XLOPER still fully supported in the new.
2. XLOPER12 integers are all 32-bit, strings are Unicode wchar_t, row and
column integers are all 32-bit integers (defined as RW and COL)
3. 2007 supports XLL worksheet functions being registered as thread-safe
with addition of '$' to the type_text argument to xlfRegister.  (There are
also new type_text codes for the new data types).
4. Those of the XLL interface callbacks that take or return XLOPERs are
supplemented by XLOPER12 versions in 2007 (e.g. xlAutoRegister and
xlAutoRegister12, xlAutoFree and xlAutoFree12).
5. There is a new FP12  floating point array structure. The old FP floating
point array structure is still supported.

You could attempt to re-write the new SDK projects and Framework library in
terms of XLOPERs, but that could be tricky.

A number of third parties who have free or commercial XLL (or XLL-related)
libraries have developed (or are developing) frameworks that will provide
support for both old and new versions.  My own book's 2nd edition contains a
C++ class that demonstrates the issues and one possible solution.

Regards

Steve Dalton