I can confirm that a 64GB microSD works. I bought the SanDisk Mobile Ultra 64 GB today, reformatted it to fat32 and it reads in my rockboxed Clip+. For some reason I get a 16 MB non-allocated part first though, may just be my bad.
Have tried to copy a file from the sansa clip+ 'internal' flash to the SD flash succesfully. Have not filled the whole SD with songs yet.
well... no. It doesn't work for me, it seems. Actually it would seem like I have 'bricked' my sdxc-card. I have tried to reformat it back but no luck. At this point, just an expensive clip-on nail.