I'll weigh in on my own question now --
I think that the recipient should write the note as promptly as possible after being notified. It's the same principle as any thank you note: you express your appreciation as quickly as you can. I have been mildly unimpressed when I gave gifts months before the wedding and never heard a word until after the wedding. The wedding date really isn't even relevant: they gave you a gift, you got it. This also serves the purpose of letting the giver know that the gift (or in this case the notification) was received. The point is, the focus is on THEIR feelings.
With all due respect to those who waited until after the wedding (and presumably at least two more weeks for printing) so that they could include a photo on or in the card -- I wouldn't do that for gifts received substantially in advance. Like, if your wedding is in September, and you receive a gift in June, don't make the givers wait that long. (For gifts received just a week or two before the wedding, it's not a big deal.) But including a picture isn't important enough to outweigh the primary purpose of a thank you note: to show enthusiastic gratitude for the gift. Putting that off for months just so that you can include a photo of yourselves doesn't serve that.
I was really surprised to see that the "Practical Wedding" site includes the misinformation that you have a year to write your thank you notes. Plenty of commenters took her to task on that one! I am guessing that this all too common belief is based on a misunderstanding of the etiquette rule that wedding guests have a year after the wedding to GIVE a gift promptly.
Similarly, perhaps the idea that you wait until after the wedding to write thank you notes even for gifts received way before the wedding day is confusion over the rule that you aren't supposed to USE the gifts until after the wedding.
As for including "We were so happy/sorry you could/couldn't join us at our wedding," although it isn't required, I would do that if I were writing after the wedding anyway, but I wouldn't put it off just for that. You just say it, and pretty much all the things Hmmm put in her post, in the future tense. I like including things in thank you notes that aren't just about you and the gift; it shows you think of the giver as a person you value, not just the giver of a gift you value. So I always include that, and, if I can, something else about THEIR lives -- "Have a great time on your trip," "Pat Fluffy for me" or "We know your roses will be the pride of the neighborhood again this year" -- like that.
I think it is really, really risky to write notes to people from whom no gift has been received (yet! some always come later) just thanking them for attending, though. No matter how you say it, and no matter how sincere your motives, there is no way that it won't come across as a nudge to send a gift, or at least make the recipient wonder if it is. If I got a note like that, I'd probably be on the phone immediately saying "Your gift is on the way! I'm still crocheting it/It was out of stock, but it should arrive soon." Because the convention is that guests write to thank hosts for hospitality, not the other way around, doing this always looks like it has an unfortunate subtext. You don't want to risk your guests' feeling like they are being called out for not ponying up faster.
There seems to be no end to rationalizations for putting off writing thank you notes! And of course around a wedding, there are so many gifts, and so much going on, that of course you can't write them all as quickly as you could for, say, your birthday; even Miss Manners admits that.
But the principle is the same: you write them as promptly as you possibly can. You want the person who was nice enough to give you a gift to know that expressing your appreciation to them is a top priority and a pleasure, not a chore.