Warning!!
Hasn't anyone considered the possibility that these mysterious "spam" deposits could be worse than spam?? Three possibilities come to mind (I received numerous mBTC myself and started searching for answers when I found this thread fyi).
I have not yet explored the Laxo and other similar spammer sites because of my concerns below. I hope you all are taking proper precautions (using computers that do not have your wallet on them, using TOR, etc.) before doing your exploration and/or hope I am wrong/unjustly paranoid ..
Some things to be worried about, for example:
1) Classic trojan attack with a bitcoin twist. When users go to the Laxo site (or whatever) they are offered downloads and/or exposed to browser scripts that are malicious. Just like trojan emails. A well designed attack using bitcoin mBTC bait would then scan for unencrypted wallets, unlocked mining machines, etc. - pretty easy to do.
IMO unlikely. People have not been reporting their bitcoin being stolen despite the fact that many people are likely visiting their site because of the blockchain spam. The only real way they could steal your bitcoin (besides running away with any bitcoin you deposit with them) is if you were to install malware and most bitcoin users are smart enough to not download something from unknown websites like this. It would really not be possible AFAIK to steal bitcoin via JavaScript or by only visiting a website.
2) Government surveillance. Let's say I am a government interested in tracking users who are purchasing illicit items using bitcoin (like Silk Road RIP). How do I find the identity of users so I can link them to transactions on the blockchain? If I were such an organization, I would need to link an illicit purchase using bitcoin to a specific real world identity. Hard thing to do if the bad actor is using TOR!! So, instead, why not seed thousands (millions?) of wallets with small BTC amounts. These micro BTC deposits would be made into wallets of known and/or traceable users (like those of us online at bitcointalk) who would then spend the btc. The spent BTC goes from user to user to user and (if enough of these "spam" deposits are made) will eventually end up in the wallets of a target bad actor providing a possible means of identifying him. As a simple example, lets say I sell 1 BTC via localbitcoins to a person live for cash. This person turns out to use the BTC for something illegal (guns, drugs, etc.) My wallet automatically chooses the inputs which happen to include one of the .00001 BTC deposits. I may not even have noticed this, but now the authorities have a way of tracing the BTC used by the illegal actor back to me. It may not be enough to get to the real bad actor, but it gives them a substantial lead (because I could give a description and details that could lead them to the identity of the person they seek). Moreover, if they "salt and pepper" the blockchain with enough mBTC, they can start to use statistical methods to zero in on suspects by tracing many different transactions that use these baited mBTC.
Any attempt to track who owns a specific bitcoin address via dust will force the attacker to guess a substantial portion of the time, making their accuracy to be low. This is exaggerated when people do not keep even amounts of bitcoin in each of their addresses.
3) Government setups/traps. Just like with "To Catch a Predator" on NBC, why not send out mBTC laced with info leading to child porn sites? Now, anyone who visits those sites can be tracked in two ways potentially (1) via the wallet address linked to an online idenity scraped from this site, and/or (2) via IP address when the poor sick schmuck visits one of those sites and attempts a download.
FUD - To Catch a Predator relies on pedos to actively seek out someone who is underage to meet up with. They do not go around offering random people a way to break the law.