I cannot give you an example, and I have never implemented such thing, but I think it's pretty fine to have a tag cloud (even 3D one) on a touch screen.
The whole idea behind tag cloud is that less popular tags (it does not depend on screen type) are smaller and thus:
- less visible
- harder to trigger (click or tap)
In other words, both visibility and clickability decrease with decreasing popularity.
While the first factor can be preserved without any problem, let's investigate the latter in the context of transferring it to a touch screen. On touch devices, to make a target (optimally) tappable, you should not go below some agreed limit. This limit is physical and is sometimes referred to ax 7x7 mm (Windows Phone, if memory serves). But it is just something agreed on, because people have different fingers, finger operability etc. Going further - producers refer to it as display size in pixels/points; for example, having just two densities for their phones, Apple refers to it as 44x44 points in their "iOS Human Interface Guidelines":
44 x 44 points is the comfortable minimum size of a tappable UI element.
Let's not fix at this 44x44 points limit, though, but let's use this points based metrics. In general, making targets smaller will make them less tappable not depending on this limit, e.g. 50x50 will be easier to tap than 44x44, while 40x40 will be still quite ok, and 10x10 will be barely usable.
Having this in mind, a question arises: should the tags' tappability decrease with decreasing popularity?
I would say it is quite aligned with the primal idea of tag cloud, which assumes decreasing the usability of particular elements depending on their popularity. However, I would put some limits to it and not go below, say, 20x20 (in Apple's metrics), totally ignoring less popular tags. In fact, this is the same way you do it in desktop tag clouds (e.g. only most popular 100 out of 20000 tags are displayed, the rest is ignored). So when transferring the tag cloud to a touch device, this limit is a thing to consider, define and test.
Another thought is that you could use other pattern for decreasing the importance of tags. As less popular tags are less visible, you can achieve (or continue it below the forementioned minimum, put by me at 20x20 points level) by operating one of these:
- font color intensivity - less important would be lighter
- font color - you could use a color palette from red to, say, blue or any other to expose/hide some tags
- font weight - less important again would be lighter (just the meanning of 'light' changes ;))
Edit (an example of the above patterns):
*The tag cloud example in Wikipedia article uses altering colors and intensivity plus of course font size: http://en.wikipedia.org/wiki/File:Web_2.0_Map.svg*