Things to be aware of when sending out electronic save the dates:
1) If the file name attached to your email includes "STD", non-wedding-savvy folks on the guest list may be a bit hesitant to open it. And you will be the butt of many family jokes.
2) Yahoo will actually freeze your account for a few hours if you try to send an email to too many people (apparently 300 is too many).
We hemmmed and hawed about sending e-STD's, but in the end it fit with the overall feel of our wedding. No frills, budget-friendly, and most important, a low environmental footprint.
Our STD's were designed by our good friend Shari Amos, a graphic designer who does wedding invites (lucky us!). I am in love with them. I couldn't fall asleep last night because the 18hr time difference meant that people might open our email as early as 1am our time. I might even sacrafice a small tree and print a few up as keepsakes... that's how much I love them.