Tech demos or challenges, like what people are submitting for 10K Apart.

Malware obfuscation.

there has to be a use case?

Of course not! But if there's no use case then this is getting an awful lot of buzz for no real reason other than Ajaxian running a story on it, considering that it's not that new. http://blog.nihilogic.dk/2008/05/compression-using-canvas-an...

It's been on reddit or hacker news before, this year iirc.

as the ajaxian article stated, i believe that people are trying to make use of it to squeeze the most out of the size-limited javascript contests that are out there. but, i've not seen an entry that does yet (haven't looked that hard though).

I think js10k limits entries to those only readable in UTF-8 which, I believe, a PNG (compressed or uncompressed) does not meet.