I've been doing stuff like this for a long time, starting under MSDOS, and you don't need to wait for the Win10 preview build to get a version of Bash. The question is why you might want to. Most of the efforts I've seen are focused on developers who want the Gnu/Linux toolchain available to use in development under Windows.
Cygwin had the intent of porting the Gnu/Linux toolchain to Win32 Rather than rewriting everything to use Windows APIs instead of *nix system calls, they implemented a POSIX compatibility layer encapsulated as a DLL. A lot of *nix code builds out of the box using GCC under Cygwin because it links against the DLL and sees the *nix system routines it expects. It works fine, but takes a serious speed hit because of the differences between how *nix and Windows do things, like the issues with forking processes. And when you are under Cygwin, you are in a *nix compatible command line environment, and must use / as a PATH separator and - as an option control character, and jump through various hoops when interacting with native commands that don't grok that in consequence.
An alternative is the AT&T UWin effort, an open source environment taking an approach similar to Cygwin in using a compatibility layer, with a port of the Korn shell as a centerpiece. (The project was run by Korn Shell creator Dr. David Korn.)
The MinGW project and companion MSYS took a different approach, building things via GCC to use Windows runtimes, and many *nix commands are available that way as well as a version of GCC configured to compile to native Windows binaries. I have it on my desktop under Win7.
The fullest implementation of bash and friends I've seen is part of Git for Windows. Install it, and you get a version of the Git DVCS for Windows and a full set of Gnu/Linux utilities in native Windows builds, including bash. Bash itself can be had from an assortment of sources, as can tcsh, zsh, and others, but those versions will be based on older code.
The interesting thing about this effort isn't bash per se - it's the addition of the libraries to Windows providing the support that makes it possible to implement bash. Right now, I'd call bash and friends a proof-of-concept demonstrating it's possible. Seeing the ability to run Ubuntu graphical apps under Windows down the road would come as no surprise. (There is a moribund project to get the KDE desktop environment under Windows using Cygwin, that actually had a working KDE desktop for Windows back in the WinXP days, so it's certainly doable.)
