Plant something? (a tree that always blooms around the time he's born...bulbs, etc.)

Some sort of time capsule or photo documentation of the day or month he's born?

I actually love the idea of asking everyone to sign a book. Happy Birthday to You by Dr. Seuss and On the Night You Were Born are great ones. :-)

Do you mean something to commemorate, or a dated gift (like the classic cigars) that serves as a birth announcement?

If a commemorative, I agree with oakbee. Plant something.

I also love the Name a Star programs.

There are also companies (RedEnvelope.com?) that do little saplings you can customize.

I've also seen "Night Sky Over..." posters, where you specify the date and place...

We had a frame with a really big mat and everyone wrote "welcome" messages to our son on it. We then put the photo from his announcement in the frame and hung it in the hall with all of our family photos. Now that he's 4, he loves to have me read all the messages that were written to him--and it's nice having it out in the open, rather than tucked away in a book. We also took pictures of all his visitors holding him, and assembled those in a book for him. It was a great quiet book for him when he was a baby.

We did personalized M&Ms when our second was born. They were a new item at the time and people got a big kick out of them.

This won't work if you want to keep the mementos but at my shower, everyone wrote notes and funny sayings on diapers. At the time it was kind of silly but once I was changing diapers at 2 am, I found them to be wonderful and amusing. In fact, my husband asked if we could get our friends to write up some more for us because he enjoyed them so much.

I've heard of families having a "birth"day cake or cupcakes.

I've seen the idea (I can't remember exactly where) but it was of a tree image made with individual finger prints as the leaves. Each person adds a finger or thumbprint to the tree and signs their name to it. I think it would be great for many occasions - wedding, new baby (visitors), family celebration. Matted and framed it would make for a nice piece of artwork in the nursery.

I saw this cute idea on Inchmark that I plan to do for my future nieces and nephews and maybe my own little ones some day: http://inchmark.squarespace.com/inchmark/2009/4/1/b-is-for-book.html

A friend makes "blessing blankets" in which you have friends email her a blessing or a prayer for the baby, and then she sews them onto little stars that lift up. You can wrap your baby in blessings every time you use it. The proceeds all go to a child sponsorship program in Kenya. Very cool.
http://www.shareinternationalinc.com/blanket.html

Over at Hoot Designs you can buy this really sweet print and have it customized with names from your family.

http://www.etsy.com/view_listing.php?listing_id=36581653

Thanks everyone! Some great ideas here :)

This looks like a great idea, for the parents anyway. People who can't come to the shower send in their advice to the new parents and those attending the party can write their personal message on a notecard. Might be a fun thing to read at the even, too, instead of the usual, "Oh, look at such and such gift."
http://www.amazon.com/Baby-Bliss-Box-Well-Wishes-Parents/dp/0307453057/ref=sr_1_5?ie=UTF8&s=books&qid=1269461474&sr=8-5

You could also have, or at least borrow an idea from, the Chinese's "life party" that is traditionally celebrated when a baby is one month old. Here is what I found real quick at about.com: http://chinesefood.about.com/library/weekly/aa012303a.htm

My husband is Chinese Indonesian and his mom threw one when our daughter was born. I don't remember it matching up exactly to what the about article says, but there's lots of ethnic groups within the Chinese population so maybe that's why or it was mixed with Indo traditions too. One thing I do remember is long noodles. There were some long noodle dishes that symbolized a hope for a long life for our daughter, probably for the guests as well. Oh, and satay with 2 kinds of sauce that were special for...something. Sorry, I was a bit overwhelmed and didn't help to do anything for the party!

I don't know if you're Chinese or not, but hey, this is America! Borrow anyway if you think it's neat! :)

A quilt, matte for a picture, or book that people can write "wishes" for the new little one.

Also, See's candy makes chocolate "See"gars. Not really inspired, but oh, so tasty.