David Thornburg apparently worked out how to get Scratch running on Linux, but his instructions are offline. There is also a workaround in a Spanish blog post. Officially, they'll have a Linux release sometime this year, if they get around to it.
I'm a bit mystified by their lack of interest in outreach to the open source community, although I guess it may be partly attributable to the fact that I don't think Squeak has been officially switched to the Apache License yet, which means the whole thing probably can't be put in a Linux distribution. Although if they're distributing an Apple licensed Squeak, they certainly don't mention it in the license information included with the Mac version, which they would be obligated to do. Squeak licensing remains an arcane mystery.