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.