Following What XML generators are currently available on PerlMonks?, you may want to use a URL like:
in reply to Newest Node alert
This will only look for nodes created in the last 0.00347 days, which is 5 minutes (the amount of time your script sleeps between polls). It's a little unnecessary load on the server, perhaps, to fetch the information for the older irrelevant nodes. Also, it eliminates the need to store a "last seen" node ID.
I also notice that your script will not announce more than one node if multiple new nodes show up in the same 5 minute window.