I use qTranslate for my bilingual site, and I am very satisfied with how it works. My suggestion is about the comments. I think it would be great if somebody makes a plugin that adds a language button to the comments so the commentators chose the language of the comment they leave.
On my website the comments in both languages I use appear under the post, in whatever language it is. Also the right-to-left and left-to-right setting of the different languages makes the comments look bad when they appear under the post whose language is different from the comment. My suggestion is a plugin that:
1. enables the visitor to chose the language for her comment,
2. applies certain style (like dir="RTL" align="right") to the respective language automatically,
3. enables the blogger to edit the language of the comment,
4. enables the blogger to chose if comments in certain language appear ONLY under the posts in their respective language, or appear under the post regardless the language.