I agree that Chrome and Firefox will support anything they can legally, but I do not think that Safari will implement Theora. Here's my rationale:
Right now, Apple sees Google as a threat, as evidenced by the recent hostility Apple is showing toward Google. Specifically, Apple's blocking of Google Voice and Lattitude on the iPhone. They are "partners" in name only.
This is because the smart people at Apple realize that Google's philosophy of inexpensive lowest bidder open platforms is the antithesis of Apple's closed, locked down, and tightly controlled vision for the future. Internally, Apple attributes their closed platform philosophy for their current successes. They realize also that trouble for Google is good for Apple.
The backdrop for all this is the entire telecommunications industry on the verge of a paradigm shift. A growing number of people are foregoing landlines for owning cell phones only. Cell phones themselves have become ubiquitous. Cell phone lag, audio compression artifacts, and frequent drop outs have reduced the phone service expectations of the general public to a point that modern voice over IP, with a modern internet connection is a valid competitor in the phone service arena.
Today's smart phones are basically VOIP clients on a proprietary, closed network (the phone carrier's network), with access to a larger, also closed, network (the international telephone system). Carriers profit tremendously from the closed nature of the network. Byte for byte, a data feed to the moon is cheaper than the text messages on most phone carrier networks. Apple also profits from this closed arrangement via its iPhone exclusivity deal with AT&T, who pays them handsomely for the privilege.
Google aims to open the phone network by implementing its functionality using open Internet based protocols. Google Talk will replace SMS messages and traditional phone calls. Other Google services will be tied in for a richer communications experience than what the telephone networks can provide on their own. Eventually, any phone with Internet connectivity will be able to use Google's services. Once this happens, the phone networks will be mere data providers for an open network, instead of gatekeepers of a closed network. This will drive down prices, telco profits, and the cost of accessing Google's services. Apple will have lost a source of revenue, as networks will not be able to afford to pay them for exclusivity.
Apple pays lip service to open source philosophies when it benefits them, but have no intentions to further these philosophies or their influence. By this, I mean that they love being able to use the work of others, and will contribute back to open source projects they've used (BSD, KHTML, etc.), but it will be a cold day in hell before we ever see an open source version of iTunes because they do not believe in the ideology. Apple is committed internally to the closed platform vision of the future, where they are the sole gatekeeper. Open formats and standards are a threat to the dominance of the gatekeeper model that Apple is committed to. This is also why we'll never see official support for FLAC, Ogg, Theora, Matroska, or any other open codec in iTunes, Safari, or iPhoneOS.