As this API is just entering public beta, it is highly advisable that you let me know if you build an application so I can contact you should any of the API details change - it's also great to see what people are building! Alternatively, you can subscribe to the blog on my website or follow me on Twitter for all of the latest updates.
This entire project came about because I created a basic RSS feed that served up the current tube status as scraped from the TFL website. As it became more apparent that people wanted more control over the data, I created the API. However, there are still a few people who would prefer an RSS feed of the latest updates rather than having to write their own application. You can get those RSS feeds here.
I have created one feed that contains the latest updates across all tube lines and then there are individual feeds for each line.