Heroku App with a Wordpress Blog
Whilst I've not used the new PHP platform I don't think what you're asking for is going to be possible.
When you deploy an application to Heroku it detects the type of application during the push process and sets up the application accordingly, Heroku use Apache to host PHP as you can see from the output in this post http://tjstein.com/2011/09/running-wordpress-on-heroku-and-amazon-rds/ - there's not been any mention of mixing platforms in a single application yet so would imagine that it's not supported.
The nearest you'd get is hosting your site and your blog in two seperate applications with the blog on blog.mysite.com and then put a redirect on www.mysite.com/blog to the blog.mysite.com url.
Im Still working on this
the closest to getting a solution is using the reverse proxy gem
see the answer here: