This lowers the bar for entry to the iOS market.
Does it also lower the bar for Mac OS X?
Can it be used to write command-line command-line applications ("scripts")? It has a REPL, which means it can do a kind of "just-in-time" compile and run. This is how Python works, so perhaps this is a viable mode for using Swift.
Via the Objective-C and C compatibility, it has full access to the POSIX libraries, as well as Cocoa, so it can clearly be used to build command-line apps. It might lack the flexibility of Python, since it's compiled. But C (C++, Objective-C) with automated memory management is still a gigantic victory for writing fast and reliable programs.
Can it be plugged into Apache to write backend applications? It's compiled, and compatible with C and Objective-C. So, one can imagine that a mod-swift component in Apache might be possible. It might be better to work through existing FCGI interfaces and write stand-alone Swift back-ends. This would require a bunch of libraries for database API's, template rendering, request and response processing, and the various bits and pieces that make up a rich web development environment. But this is largely available for C and C++, making it available to Swift-based backends.
Is one language even a desirable goal?
The idea of having one official version of the class definitions seems very helpful for capturing knowledge and managing the intellectual property that is embodied in application logic.
The Swift Programming Language