WordPress.org

Will Wp2BB create duplicate content that might be penalised by Google?

It could theoretically happen if you choose to include the WP post text in the correspondent PhpBB topic, but this is fully configurable in the settings ("Topic message text" option). It is definitely better to create PhpBB topics generic text such as "Discussion about the post 'xxx' in the blog", just using the WP post title

Can I use images/fonts/colors in the PhpBB topic?

Yes, the "Topic message text" setting will accept HTML and BBCode

Can Wp2BB sync registered members in WP/phpBB with single sign on?

No, Wp2BB does not handle users in any way. This feature could be implemented in the near future. In the mean time you might want to check WP-United. Goes pretty well with Wp2BB .