Enter wowrealmstatus.net. It's a simple service: it sends you one email when your realm is brought up. (It can also be set to send you an email when your realm is brought down, or when its load is low, medium, or high, but I have yet to find a use for this functionality.) Enter your email, pick your realm from the menu, click the "Alert Me" button, and you're done. It only ever sends the one email; if you want another alert in the future, just set it up again then. Simple, effective, not annoying: a great web tool.
And if you didn't know already, your mobile phone almost certainly has an email address which translates to text messages (AT&T phones, for instance, are [phone number]@mobile.att.net, like firstname.lastname@example.org), so you can get alerts that way too, though your carrier will bill you for it if you don't have a text plan.