Valve to implement new security measures for Steam starting next week
Steam still has a ways to go in terms of functionality (mostly to adjust things they implemented, like Greenlight, mods, and trading) but a new change is coming on March 9, sprung by security concerns.
For starters, you can't dupe items anymore by trading. People would previously do this by trading an item away, then put in a claim ticket to restore the item. That seems like a no brainer. Also, the community market will have a 15 day hold, as will trade holds in general unless the individual has been on your friends list for a while.
So yeah, not huge, but for all you heavy traders out there, it's something to be aware of.
Security and Trading: Update [Steam]