Using Photoshop is an essential part of texture painting in 3D programs.
It would be wonderful to have the functionality to import a UV map as a unique layer, which retains the original 3D UV information.
This would allow the ability to paint across their shared edges, thus creating seamless textures.
Currently, the only way to achieve a seamless texture is through a 3D program, painting on an actual model.
One would think this could be achievable painting across a 2D UV surface.
I've done this in 3dsmax by going to my unwrap uvw modifier, clicking "edit" and then in that dialog hitting "tools>render uvw template" and exporting that as a tif with alpha and using that as a template layer in Photoshop. This works pretty well if you want to work in flat 2d type space.. which I often like to do.
As far as a more customized painting envioronment for painting directly on 3d models I know "BodyPaint" by Maxon is pretty good. ( http://www.maxon.net/products/bodypaint-3d.html).
I think RightHemisphere used to make one called "DeepPaint" but I don't know if they still make it. I've also heard mention of http://www.blacksmith3d.com/ as a painting app but never much looked at it.
I don't know about other 3d apps and how they export uvw templates but it shouldn't matter that much if you have CS4 Extended because you can import models with texture coordinates in place.
I've actually never used the 3D functionality in PS because I do mine in flat 2d space most of the time and just keep checking MAX but that's probably just old habits dying hard. From what I understand you can simply click on the layer (ie diffuse, specular, etc) and edit in flat mode.. course I don't know about that showing wires.. that's why I use the "Render to UVW template" so much. I used to just screen cap it and key out the background before that feature. Perhaps that would work in your 3d software if it is not MAX. Hopefully this is helpful. I'm not really that hip on Photoshops 3d capabilities. Like I said "old habits die hard"
There is a nice tutorial at this link however: http://www.photoshopcafe.com/cs4/3D.htm