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You can do this via .sublime-settings files: this is how the "View/Syntax/Open all with current extension..." menu operates. By placing "extensions": ["txt"] in Python.sublime-settings, for example, .txt files would be opened with python syntax highlighting, taking precedence over what's specific in the .tmLanguage files
New versions are released weekly during the alpha - you may want to make sure you aren't using a dev version, which is released more often
The version of libgio on Ubuntu 10.04 is too old. libgio is part of gnome, and can only be upgraded by moving to a newer version of Ubuntu - 10.10 is required.
It requires a specific version of libgio to work - if you run from the command line, a message will be printed to stderr if single instance support is disabled
In general, it will happily install over the top of the previous version - there's no need to uninstall the previous version first.

The only issue is the one you ran into, which occurs when Sublime Text is currently open. This would also be as issue for an uninstall.
Sublime Text 1.4 has a (context aware) spell checker, and it will be coming to Sublime Text 2.
"isn't actually useful" is overstating the situation, but you're absolutely correct about the current presentation not working well with minimised windows and exposé. I'll change it for the next version so the project name is included in the regular title text.
Plugins can define key bindings in their own packages. The file must be named "Default (<platform>).sublime-keymap"
Every key resonable key should now be bindable on OS X and Linux as of build 2046