First off, I really love the idea of automating page tests. It looks like you have a great start for doing just that.
Currently, when I run a test, I look over the results and pretty much never come back to it ever again - at least no more than a week.
I would love to see hourly, daily, weekly, monthly, and yearly trend data for a particular webpage or even an entire website. I realize this could be done manually but it would be very time consuming to do - especially when the entire process can be automated.
The first thing I would focus on is getting the testing process using a database instead of files. This way you have better control over the data.
The test history page could be setup a bit differently. You could have unique URL links (or titles such as My Homepage) on this page. Click one of these links could open up a new page that lists dates and times that tests were run for this particular URL. You could have links here to view hourly, daily, weekly, monthly, and yearly trend data if enough data exists for these particular URL. You could also have a link here to run test now so the testing data is already propagated into the test form.
On the actual test form, you could have a drop down box thats labeled something like “Run Test” and then “Once, Hourly, Daily, Weekly, Monthly, or Yearly”. You could have this option only available to registered members so it would encourage people to register.
If a test fails to run successfully for various reasons, you could have an automated email setup that informs the website owner that something went wrong with the test. This could be an indication that maybe their website is currently down and they need to fix it.
For the trend data, you could have flash graphs where data points are represented by circles. Clicking on a circle could goto the test results for the corresponding point.
Let me know what you think.