Why not write a script that checks the site from a cron job on a different server. You can have it send an email or text via twitter. Cron is a simple service. And if you can write a script that emails you or your phone, you only need to place it on a very reliable server or 2.
I wrote a script, run via cron, that checked a ticket web page every 30 minutes for concert ticket availability. If the text changed, it implied that the tickets were available; the script sent an email to my phone, a text message, telling me call for tickets. ( Ratatat was the band )
pingdom checks your site from multiple servers all over the world. they provide great uptime and response time graphs. lot more than a one line script can do via a cronjob.
I wrote a script, run via cron, that checked a ticket web page every 30 minutes for concert ticket availability. If the text changed, it implied that the tickets were available; the script sent an email to my phone, a text message, telling me call for tickets. ( Ratatat was the band )