That's not entirely correct... Dreamweaver CS4 does not use Rosetta at all on Intel based Macs - Older versions such as Dreamweaver CS3 had to run Rosetta which was compiled for PowerPC chips... this isn't a Carbon vs. Cocoa issue. This had to do with the binary that was compiled, instead. In fact, I am running Snow Leopard and don't even have Rosetta installed (you have to answer yes to dialogue box to install it when Snow Leopard encounters a program that requires it). Rosetta is the emulator needed to run code compiled for Power PC chips and does not directly correlate to whether Carbon or Cocoa is is used.
The Carbon vs. Cocoa issue that Adobe had was that CS4 had already been near finished when Apple announced it was going completely 64 bit and scrapping Carbon in the future in favor of Cocoa. So while Adobe brought 64 bit support to Photoshop CS4 on Windows, they did now do so with the Mac version... that is until when CS5 gets released which will put the Adobe products in Cocoa on the Mac, as well as, making them 64 bit apps. But the Carbon language has no problem running on the latest and greatest Intel Macs - the only issue was that 64 bit apps in Carbon would not be exposed by the new OS "dumbing them down" to 32 bit.
There was a great article about a year ago on this when Apple announced it was eventually go all 64 bit: wwdc-2008-is-mac-os-x-106-the-death-of-carbon/
So unless your specifically telling your OS to "Open using Rosetta" (which you would have to check the box in the Get Info dialogue box), Rosetta should not be needed for any of your Adobe CS4 apps. In fact, while Snow Leopard has gone completely to 64 bit architecture and Adobe is still 32 bit, I have noticed several performance gains in terms of speed in both Dreamweaver and Photoshop (probably because other tasks running in the background are made easier).
But Dreamweaver CS4 and Extension Manager ARE compatible with Snow Leopard... so I am not sure what you are doing on your system (maybe you want to check to make sure that the aforementioned check box for "open is Rosetta" is UNCHECKED.
As far as WebAssist extensions are concerned, I have all of them running on Snow Leopard. I have not used Google Maps, though, since my Snow Leopard upgrade so I cannot determine the severity of that issue. But, I have used nearly everything else (SecurityAssist, DataAssist, iRite, FormBuilder, Cookies Toolkit, etc.) and have not had any issues.
Happy developing,
Brian
