Although patches recently went into the kernel to change block containers to unsigned types, arithmetic on those types can still cause overflows. This is immediately obvious at mount time, when attempting to mount a filesystem with 2^32-1 blocks.* Care must be taken when iterating over group descriptors, and calculating group descriptor count, to not overflow towards the end of the filesystem. These patches are now upstream in the -mm kernel, for ext2 and ext3. *actually creating such a filesystem requires changes to e2fsprogs, addressed by another bugzilla entry, still to come.
And for the record, said patches are working on two test boxes I've got. # df -h /mnt/sparsefs/ Filesystem Size Used Avail Use% Mounted on /dev/mapper/sparsefs 16T 25G 15T 1% /mnt/sparsefs
in fc6 stream now.