Hi Hoan,
I have read your patch and it looks fine so far. I will test it on cmucl later today, and if things go well, I'll commit it to the trunk.
I have not made much use of floats, so I cannot comment on the current implementation much. I would also prefer not to have this workaround for OpenMCL, so if you could come up with a patch, I would propably add that as well. The new implementation for %encode-string looks fine, and as far as I can see it is more consistent with the %decode-string implementation.
Thanks a lot! -Hans