Give it until August. There are still some things that cause warnings in rails for 1.9.1, but otherwise the small speed boost is always helpful, and there some more delicious syntax stuff as well, like hashes having JSON syntax. This is awesome because you can switch between Javascript and Rails without typing => in your js files.
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1.9.1 still has quite broken items, and 1.9.2 has even more broken stuff as it's very much still in development. This is why people haven't switched.
That said I'm starting to see people use 1.9.1 syntax in tutorials, tips and blog posts.
Give it until August. There are still some things that cause warnings in rails for 1.9.1, but otherwise the small speed boost is always helpful, and there some more delicious syntax stuff as well, like hashes having JSON syntax. This is awesome because you can switch between Javascript and Rails without typing => in your js files.