A WebmasterWorld thread reports Google not only bolding the ad text that matches your exact search query, but also bolding variations of that query.
For example, a search on ipod in singular form, also bolds the word "ipods" in plural within this Circuit City ad.
WebmasterWorld member Rehan confirms that he also believes this is new, saying;
I'm pretty sure this is relatively new, or at the very least the broadening of what may have been happening on a limited basis before. A few weeks ago I split up a "Keyword widget" campaign into separate "Keyword widget", "Keyword widgets", "Key word widget", and "Key word widgets" ad groups just so I could get bolding without using dynamic keyword insertion. With this new change, I don't need to do that because "Keyword widget" in the ad text is bold even if "Key word widgets" is the search query.
I personally do not know for sure if this is new, but it seems like the majority of the AdWords professionals at WebmasterWorld seem to believe so.
Forum discussion at WebmasterWorld.