I'm planning to order ours from shutterfly after the baby is born. With our first, I handmade announcements with scrapbooking-type stuff, and although I had all the envelopes addressed before she was born, assembly took some time and my in-laws ended up helping not so much because I asked but because they happened to be here when I was doing them. They thought I was NUTS.
This time around, with two kids to worry about I decided to go the pre-made no-assembly route. Since I won't have the envelopes till after I place the shutterfly order, I bought sticky labels - the slightly larger shipping labels - and hand wrote all the addresses out on the labels. I have a hangup about hand writing addresses on stuff like holiday cards, invitations, birth announcements, etc. Even though it's all in my computer and I could print it out in a heartbeat, I just can't bring myself to do it that way.
I also went ahead and bought stamps, and it's all sitting in a folder ready to go. USPS has really cute storybook character special edition stamps out right now - I ordered mine online. Also, if you haven't seen this, you should check it out
http://www.pictureitpostage.com You can buy real postage stamps with your own photo on it - you just have to upload a digital photo. They're more expensive than regular postage (about $0.70 per $0.39 face-value stamp) but it would be really cool if you had the right photo and the extra cash.