I have three different Gmail accounts:
1. Standard @gmail.com account -- shows the FireGPG buttons just fine.
2. Google Apps (hosted @domain1.com Gmail) account -- shows the FireGPG buttons just fine.
3. Google Apps (hostes @domain2.com Gmail) account -- buttons do not show up.
I have tried enabling and disabling all sorts of plugins, it had no effect. I am using the same browser to log in/out and check all three accounts. (Firefox 3.0.6). I did used to have FireFTP installed, which caused some interesting error messages, but I have since removed it; restarted the browser.
Open to suggestions. If you want me to test something, let me know.
Jake