Since installing Mobile joomla my site has now got 100's of duplicate title tags and duplicate description tags in google webmaster tools - Duplicate meta descriptions 2,014 - Duplicate title tags 2,007 and example is
www.thewedding.uk.com/photographers/cumbria/aperture-photography.html and www.thewedding.uk.com/photographers/cumbria/aperture-photography.html?device=desktop - how can I fix this so ?device=desktop is not displayed ? or has a diffrent tag line?

Have the same issue and also want to know how to fix it.

Well... go to google webmaster tools and select the questioned website you have this problem with. From the left menu choose:

Site configuration>URL parameters

Then, just find the "device" parameter and click edit on the right. Tell googlebot that this parameter changes the content and not to crawl urls with this parameter. Markup links are no-follow, anyway. I don't know why this happens in the first place. I have this problem, too.
If a mobile user finds your website in google search, when they open it, if you have set up the MJ right, they should get automatically redirected to the most suitable version for their device. Beware of htaccess or webconfig misconfiguration in your website directories. I guess that's it.

If you guys have a better suggestion, let me know.

"?device=..." urls come from Select Markup module, but this urls have rel="nofollow" attribute, so I don't know how it's happened that urls have been indexed by Google.
It's not a problem for mobile versions of site (that is "?device=iphone" etc), because of they have rel="canonical" tag. But "?device=desktop" urls really can be indexed by Google because of MJ doesn't affect desktop version of the site.

you can try to echo the website index instead of device desktop

Have you guys resolved this issue?

well... yes... just tell search engines not to crawl urls with that parameter

Thanks you bud.

I have one more solution realated to this
If you add rel="canonical" labels throughout your website, it will eliminate the ?fontstyle=f-smaller" design URLs from the look for catalog.

Once they appear to have been eliminated, which you can examine using a look for like [site:www.domain.com inurl:fontstyle], I would suggest that you prevent that design of URL in your spiders.txt to quit google from moving those ineffective URLs.

I have the duplicate tags and I see ?device=xhtml in webmaster tools...

where does xhtml point to?

Can I tell webmaster tools not to index it? Will there be a problem?

Any advice appreciated.