When using Joomla's cache, the mobile version of the site is served up to non-mobile browsers. This is the same issue I had with the PDA plugin. Clearing the cache totally solves the problem, until 15 minutes later when the site caches the mobile version again.
Here is the workflow I used to recreate this issue:
1. Login with an iPod - browse around a bit and leave it logged in
2. Then, login with Firefox on a PC with the User Agent extension installed, set to Blackberry 8703. Browse around a bit.
3. Login with Safari on a Mac ***so far, everything is fine**
4. Wait 15 minutes
5. Go to yet another PC and startup Firefox AND Chrome.
6. Go to the site with both browsers
7. Watch both PC browsers get served up the mobile template
To fix the problem: (again, I can recreate this every time)
Login to Joomla admin area
Go to TOOLS - Clean Cache
Select all the cache files and delete them
Go back to the PC running Firefox and Chrome and refresh the page in both browsers
The right template is displayed
(system Plugin Cache ENABLED - Also in Global Config, Cache set to YES, 15 mins and FILE)
(User agent checking set to WURFL)
Joomla 1.5.14
PHP 5.2.8