I love the careful wording on Adobe’s Alchemy site:

The purpose of this preview is to assess the level of community interest in reusing existing C and C++ libraries in Web applications that run on Adobe® Flash® Player and Adobe AIR®.

It needs to be put cautiously, because Alchemy – unlike Google Web Toolkit, which handles Java to Javascript – is not intended to become a general-purpose alternative to ActionScript. Rather, the idea is to enable existing C/C++ libraries to work within Flash applications. Examples given include transcoders, cryptographic algorithms, and XML parsers. Performance is said to be 2-10x slower than native code. Alchemy was demoed today at Adobe’s MAX conference.

Possibly a Java to ActionScript compiler would be more useful to most of us; though for new code ActionScript 3.0 is close enough to Java that it should not be difficult for most developers to learn.