It should support TrueType fonts - under the hood it uses FreeType, which supports a lot of formats, just not *all* of the ones in Windows. It is by no means limited only to Arial.
However, you also need to be sure that the browser has the font, which means you should probably stick to the standard web development fonts like Arial, Helvetica and so on. HTML5 also has downloadable fonts support but that's not something on the immediate agenda to implement.
The plugin system allows anyone to make any kind of plugin. Even if the Text object only ever used fillText, someone can make a plugin that uses bitmap fonts. There's no reason to be limited by official plugins.