I like what the go authors think about dependencies being backwards compatible, but not everyone thinks that way, and let's face it, people make mistakes and make incompatible changes unintentionally.
I started a project last weekend (still a WIP), go get version[1], that basically proxies the `go get` commands, but fools it into thinking the master branch is either a tag or a version to get around some of these problems, while aiming to still work with the go tool.
For example, if you have a tag named v1.0.3, you would change your import path to be something like this:
gogetver.com/github.com/user/project.v1.0.3
There's also gopkg.in[2] which does something similar, although it appears to only work with github.
[1]: https://github.com/rsenk330/gogetver [2]: http://labix.org/gopkg.in