I've been working on adding some elements like tattoos or text to some textures for my figures and their clothes, but I'm having a tough time dealing with the distortion caused by the planar projections of the UV Maps.  Are there tools that people (merchants especially!) use to apply these types of elements to their textures and take the distortion into account?

Easiest way would be with 3D painter like 3dcoat or bodypaint. With one of those you would just place your texture/text onto your model where you want and it would automatically compensate for the distortion of the UVs. If you do texturing commercially, then you should be buying one of these anyways as they also help in fixing seams and in hard to texture objects like clothes or hair.

If you only have a 2D editor like photoshop, then its a case of using the warp tool with a lot of patience and testing.

I'm a bit confused by your mention of planar mapping, since I don't know of any modern figures or clothing that use it (although there may be some freebies, perhaps). UV mapping tools have advanced to the point where nobody needs to accept a poor map any more. Be that as it may, distortion and seams are still a pitfall for the texture artist. I've used Decal Master a couple of times, and found it to be an affordable alternative to the 3D painting apps which can be expensive for a hobbyist. Blacksmith3D have a freeware version of their 3D painting program available too - and if you're in the market for the full-strength version, I've heard good reports of their suite.

in modern poser figures, there's some edge-stretching on UV-unwrapped maps, hence if said tattoo is away from a texture edge, just add it as a multiplying overlay with the skin texture, without too many worries about distortion.  in addition to bob's linx, there are sevl. threads in this forum on how to apply a small tattoo to a skinjob in poser.  advanced topic.

Thanks everyone for their responses so far.  Believe it or not, Miss Nancy, I did do a search before I posted my question - several searches in fact (texture mapping, tattoos, decals, distortion, uv map, making textures, etc.) and came up with nothing of use.

As for planar mapping, perhaps I'm calling the projection of the unwrapped mesh by the wrong name, but V4's is not the easiest UV map to work with for a beginner.

I tried out the demo of 3DCoat, and perhaps I'm not using it right, but the results are far less than what I'm looking for.  I was trying to use a hi-res PNG as a decal of sorts to apply to a texture on V4 that I already have using both the paint tool (following the paint picture tutorial on their site), and the spline picture tool (using the paint patterns tool).  Both came out very splotchy and uneven and blended poorly with the original texture underneath.

The link to DecalMaster looks very promising, and the price isn't bad either.  I'll be checking out Blacksmith3D's offering tonight.

All tips, suggestions, insights, and directions to tutorials or tools are still very much welcome!

o.k., in addition to the above, carrara 7 allows one to paint on a poser model, and CS3E (or CS3 used concurrently with poser) allows one to paint on a texture and see the results on the model almost instantly.

Just a note that I believe 3D painting is available only in the Pro version of Carrara 7.

Photoshop Extended's 3D capabilities are considerably higher in CS4 than CS3.

There are a number of tricks you can use. Programs like Zbrush can easily deal with this sort of thing but, if you don't have Zbrush, it is possible to to use a normal 3d program that has texture baking and camera projection capabilities. It gets a bit tricky there, though.