Certainly others have thought about this, but I don't see the idea widely spread or at use, so here goes my version/vision. (Warning: shameless namecoin advertising)
A big problem for the smaller exchanges and potential decentralized exchanges is the legal difficulty, cost and long time to transfer fiat money. It hinders arbitrage operations and accessibility of small exchanges in general.
Being able to transfer bonds of EUR, USD and the like via cryptocurrencies could help the situation. It would allow to separate the exchange process from the process of getting fiat money into the chain so both processes could work more efficiently. (Something similar is coming up by using redeemable codes - there should be some advantages by doing it via the blockchain, though.)
While marking bitcoins is impossible, it would be very well be possible to follow a coin as long as it never gets mixed up with other coins. Also a rule to traverse the chain ("last out") could be established to determine the validity of such a marked coin and in effect track it. Such marked coins can then be proclaimed to be bonds (bitbond / cryptobond?) It should even be possible to send out dividends to the bond holders.
To be able to handle this it is important to ensure separation between normal and marked coins (bonds).
Example: I register bitbond1 and sell it for $1. Whoever transfers the bond back to me I will pay $1 back.
But this goes much further than transferring fiat money. It could be used for gold, stocks, options (bitcoin options or other options from stock markets), public registers. I could create a bunch of bitbonds and proclaim each a direct share of a company. Dividends would be paid to the addresses holding the bonds/shares.
I imagine a bitcoin / stock market gateway. Some day everything will be in the crypto chains.
Adding a holder-only modifiable text to the marked coin enables various more possibilities.
Shares could then be used to vote and be issued to all kinds of groups. It would allow for a permanent or timed vote (block height). Also votes could easily be redirected on the fly to other people / political parties.
Shares/bonds could be linked/chained to ensure validity (and do more? form groups like a swarm?).
Funny thing is, most of this is already possible with namecoin today...
1. Register a couple of domains (or other namespaces)
4. return / dividend / vote / ...
These are good times. I am glad to witness the beginning of the cryptochain age.