If you plan to run them in a 64 bit host: Yes. Host and plug-ins bit depth must match. Same for system kernel and drivers (kernel extensions): A 64 bit kernel needs 64 bit drivers and vice versa.
But: An OS X system doesn't necessarily have to be entirely 64 bit. You can run 64 bit applications on a 32 bit kernel (thus using your existing hardware and drivers with a 64 bit DAW and 64 bit plug-ins) and vice versa. Or said the other way: You will be able to run 64 bit applications and plug-ins on a machine that isn't able to boot a 64 bit kernel (which surely is good news for owners of slightly dated machines).
I can see this scenario before me:
Apple releases Logic 64-bit.
Since there are no 64-bit AU's (actually, there is 1, Stylus RMX)
Basically no-one can use it in production, and we have to use Logic 32-bit for any serious work.
So most AU developer will wait and see if they "need" to make 64-bit versions, because "everyone" is using Logic 32-bit.
So the 64-bit transition will take a looong time...
My guess is that Apple is in touch with some of the biggest AU developers, say Spectrasonic, NI, IK-multimedia, Waves East/West, UA and so on...
Lobbying them, maybe even helping them to make solid 64-bit versions, and when most of them are ready, Logic 64-bit and bunch of the most common AU will be released simultaneous, or pretty close...
Now this will put pressure on those developer who haven't gone not 64-bit native, since now we "almost" can do everything in 64-bit and those plugs that strays behind might get replaced by a competing product.
Well, since SL is just shipping You have to get the 3rd-parties some time to do their work.
Trust me, doing application development on pre-releases of an operating system, while it's possible, it's not a recipe for stable and robust apps...
And don't forget that some of the biggest AU developer actually are more of a Windows shops, and probably could care less of what Apple wants...
See how long it took for some of them to get their AU to work right, I guess that some of them are depending on "wrapper" code that are licensed from 4'th party developers that has to be rewritten for 64-bit.
Some of them still wraps VST's at runtime, and I don't know were Steinberg stands with their implementation of 64-bit VST SDK os OSX?
Just saying, this is not a simple task. It's a lot of things that needs to come together simultaneously for it to be useful at all...
Not on the Mac as far as I know but on Windows SONAR has been 64-bit for several years now. They also provide a wrapper that allows (most) 32-bit plugins to run in their 64-bit host. SONAR is actually an excellent DAW, easily one of the best, but still I have migrated back to Logic.
Bottom line, users of large sample libraries have reason to look forward to true 64-bit but for me (and most of us?) there are more important issues that drive our decisions.
64-bit executables and 64-bit audio path are two separate and independent things. SONAR for example has two separate executables: one for a 32-bit OS and the other for a 64-bit OS. Both versions have a switch which lets you choose whether you want a 32 or 64-bit audio path. In other words, you can have 64-bit audio on the 32-bit version of the program or 32-bit audio on the 64-bit version if you want.
The main advantage of 64-bit audio is accuracy since 32-bits still gives you lots of headroom.
P.S. Sorry to go on so much about SONAR but I use it as an an example simply because I am very familiar with it's details. Having recently returned to Logic I'm not that familiar with it's internals yet.
Close to a thousand db of headroom at 32 bit. 64 bit is not going to make any difference to the sound, just the size of the addressable memory space. You can easily beat the 4G limit now with apps like Bidule, that is, if your music is really compromised by the 4G limit.