It's a really difficult thing. According to the very strictest etiquette, even the existence of a gift registry is A Really Bad Thing. (It's correct that registries were the inventions of merchants -- some enterprising china merchants started keeping files of the china patterns of their patrons when people wished to buy and give china, and it all snowballed from there.) However, etiquette is all about getting along in one's own society, not the society of someone else's society. In some society, it's perfectly acceptable to put gift registry cards in the wedding invitation. In others, it's anathema and is regarded as telling people "don't show up without a wedding gift"! You have to know what's acceptable to your own society and in your circle of friends.
That said, if you want to be truly safe, never put registry cards in ANY invitation. If you want, putting them in a shower invitation is widely acceptable. The given reason that it's okay in shower invitations is because a shower is a party that's held for the sole purpose of receiving gifts (which is also why the bride and her attendants must always work VERY hard at making sure no guest is ever invited to more than one shower). Showers, strictly speaking, are actually against all etiquette, because the idea of a party at which one must show up with a gift is considered against etiquette.
And though it's acceptable in some society, I personally loathe the idea of asking for money, especially ON the wedding invitation!
And yes, the best way to spread word of a registry is to tell your wedding party and your family where the registry is, and they should only mention it if someone asks them.
I used to work for a registry department of Bed Bath & Beyond, in fact I opened the department for the store and was the lead consultant. It was one of the toughest balancing acts ever. I regularly fielded questions from both brides and grooms and guests about what was "proper". (I particularly remember one woman who was invited to the wedding of her extremely wealthy family -- she was sooo stressed because she had almost no money and she'd been invited to, count them, EIGHT showers.) It's awful how much stress can be built into something that's supposed to be a joyous occasion.
We were often stuck in the embarrassing and awkward position of trying to let people know that putting the registry cards in their wedding invitation was against strict etiquette at the same time one tried not to insult a society who thought it was perfectly acceptable. (Most wedding professionals feel they must warn brides and grooms because of the number of people who have put registry cards in invitations only to get hammered on by a mother or grandmother or aunt or whoever, and who usually come back angrily asking why they weren't warned it's a practice that's considered "against etiquette.")
I personally am in the "personal gift" category. If I don't know a couple well enough to know what they might like, I generally give them a set of two Tiffany wine glasses. (I don't know if it's the famous Tiffany blue box with the white ribbon that people remember or the quality of the wine glasses, but I'm usually fervently thanked.) It's rare I ask about gift registries. But they can be handy.
It should also be remembered that technically guests have up to one year AFTER the wedding to purchase and send gifts, so if you buy things on your registry after your wedding, make sure you have it marked on the registry. Oh yes, technically, gifts shouldn't be brought to the ceremony or receptions either, but they almost always are, so brides and grooms must be ready to have someone standing by to take, store, and transport gifts. (Gifts are 'supposed' to be sent to the home of the bride before the wedding and to the home of the couple after the wedding.) Oh -- and of course, no guest should EVER be made to feel like they must give a gift. EVER.
I'm from a Chinese background. We have our own version of the money dance; during the reception as the bride circulates among her guests, it is auspicious to hang gold in necklaces and bracelets upon her as part of her dowry: it is meant to symbolize and start off the marriage with good financial fortune. (Very pragmatic, the Chinese, as are many different cultures!)
A registry could contain items that are suitable for the showers (a spatula! a pot holder! a colander!) to regular household items (towels, sheets, every day stoneware) to gifts that are meant to last a lifetime (silver items, special occasion china, crystal, art). If you include items from a few dollars to about $75, that more than covers the range that most people will want to spend -- about $35-$65 is the most normal for wedding gifts, where shower items can cost anything from a couple of bucks to roughly $30.
Often registries allow for notes that are printed out with the registries. We often included notes about colors and styles that the couple planned to decorate with so that guests could choose gifts not on the registries and know that the colors would work in the new home the couple would make together. We also would include notes about other registries -- and even occasionally notes on the fact that the couple didn't want gifts!
