Digging a little, I found that is possible to use this functionality also on Linux, in my case Ubuntu 8.10 Intrepid Ibex. As far as I tested, this method offers only audio (voice) functionality, because of some Wine limitations.

For this to work, you’ll need the browser plugin to be installed on Windows.

What you have to do is:

1. Install the latest version of Wine.
2. Download Firefox 3 for Windows and install it under Wine. LE: also install Adobe Flash Player
3. Modify this file, replace alex with your username.
4. Using wine’s Registry Editor (~/.wine/drive_c/windows/regedit.exe) import gmail.reg.
5. Copy from a windows installation the next files from Documents and Settings (preserving directories structure) into ~/.wine/drive_c/windows/profiles/alex/ (replacing alex with your username):

/Local Settings/Application Data/Google/Google Talk Plugin/googleadapter.dll
/Local Settings/Application Data/Google/Google Talk Plugin/googletalkax.dll
/Local Settings/Application Data/Google/Google Talk Plugin/googletalkplugin.dll
/Local Settings/Application Data/Google/Google Talk Plugin/googletalkplugin.exe
/Application Data/Mozilla/plugins/npgoogletalk.dll

Open Firefox from Applications->Wine->Programs->Mozilla Firefox and check if the Google Talk Plugin NPAPI appears installed (Tools->Addons->Plugins).

If everything went right, you may now happily use the voice capabilities of the new Gmail Videochat feature, on your Ubuntu Linux Installation!

LATER EDIT: Because of the popularity of this post (+1000 views), I also made a small script doing all of this for you, you can download it from here and use it at your own risk.