Twitterhawk is a service that flags tweets based on your keywords, and allows you to auto-respond - or respond later, by hand - with your marketing message.
From the home page:

TwitterHawk is a real time targeted marketing engine that will find people talking on twitter now by your chosen topic and location, allowing you to really hit your target mid conversation with ease.

It will periodically search twitter for you and either auto-reply or generate a list of matches for you to respond to or reject from your twitterhawk account.

Seems the best thing to send in your tweet would be a link to your site or service.

Here's a use case from their website:

Let's say you just opened a new coffee store in Queens and wanted to let people know about it. As part of your advertising efforts, you could setup TwitterHawk to search for things like "coffee near:Queens within:8mi" (of course you could simply search world wide if you are global).

We would then periodically (at a frequency determined by you) find twitter posts that mentioned coffee by users that are actually located within 8 miles of Queens such as
'@cracksh0t Oh I could really go for a coffee right now' or
'@loxly Coffee... my one true love'

Depending on how you set your search up, the system will then either send the response automatically right then, or it will add it to your matches list for you to check over and confirm.

Should there be a match for coffee on something like '@coffeeh8r I cannot stand coffee!!!' you can simply remove this from your matches list.

Twitterhawk can track how many links you send and how many of those result in click-throughs - sort of a CPC model for tweet-marketing.

So - is this spam? Or smart context-sensitive communication?

Twitterhawk's services page

The excellent MIT Advertising Lab blog has this example of Photochromic ink used in a DM piece:

Another example of the message being built into the medium -- a print ad for Sundown sunblock, one half of which changes color when exposed to the sun (see full creative here). If you know how they might have achieved the effect, could you please leave a comment?

Why, don't mind if I do:

Photochromic ink is a technology that I saw paraded around once a year or so when I was in Print Production. The related Thermochromic (heat-sensitive) inks are a little better known, being used in things from kids' snow boots to coffee mugs and beer cans, but photochromic has never really gotten going.
It's not cheap, especially for a DM piece that I imagine had a budget of under $1 a piece (just a WAG, but you know...).

Nice to see someone got a project out the door with this, anyway.

Adverlab article

Here's a manufacturer's page.

Well!

I've certainly been gone for a while.
After a great bloodletting at the agency I was working for, I'm now out on my own - and spending most of my time trying to get a new business going.
While it's still not really ready for public viewing, a small site is up - and I've committed to get back in here and post some more cool stuff for marketers.