Post in multiple blogs

I write multiple blogs. Many of my articles that I write over lap. What I would like to do is be able to blog in one location, and have the posts show in the specified blog(s).

I thought I could just specify what category each post is for, and each category would be for a blog. So if I wanted a post to show up in multiple blogs, I would just specify multiple categories.

I have a couple of ideas, I was hoping someone might help lead me down the right path.

1. Use ONE theme with a different view for a certain category.
*Problem: How to do I tell it what theme to use if the article is a part of multiple categories.

2. Use two different installs of WP but one WP database.
*Problem: If I set the current theme in one it will change the theme in the other install.

3. Use some type of posting plugin/third party app that allows you to post to multiple blogs at once.

4. Have one blog and aggregate an RSS feed to the other sites.