Building up P4, and noticed this cable rub is going to happen on the downtube near the bottom bracket -

Do people normally put cable liner here on the derailleur cables or something else or just leave it like that?  It seems like the potential for wear and tear is there.

I can't say for sure, but I'm guessing those exposed cables should have a section of cable housing on them at that point. Otherwise, as you say, the cables will quickly carve grooves in the frame.

My P4 rubs, I just ride it.

 Gore fully sealed housing includes full-length liners; might be interesting to try those.

Or a couple pieces of electrical tape.  When the tape wears down, replace with some new strips.  

You could also use those clear frame protector adhesive stickers, but since this is a spot on the underside of the frame, black electrical tape is probably easier to find and cheaper

 I had the same issue with my 2009 P4 and I don't think the 2010 P4 is different; only the internal cable routing is simpeler on the 2010 P4, since it uses the improved ICS3.

I was no problem in my case. What i did is use a piece of outher plastic cable on the front derailleur inner cable, like used on cyclocross road bikes (not normal outer cable which is to thick but thin dirt protectors). Only the front inner derailleur cable really rubbed on the downtube of the P4.

 Even without  'a solution' i found it really isn't a problem or issue and i think you can ride it 'like it is' without any problem.

 Enjoy,

EJ

Assuming the nylon cable guide is attached to BB shell with a screw and not fixed, then simply take the guide off, and with a round file or your drill make the screw hole into a slot that will allow you to move the guide forward. It looks like all you need is a few millimeters in order to "lift" the cables off of your frame. Another solution might be to simply wedge a small spacer under the leading edge of the guide, again, in attempt to lift the cable a bit and get it off of your frame. Either way, your objective is not only about saving the paint on your frame, but also to reduce cable friction. Thus, the electrical tape solution (that others have suggested) is too temporary and the cable will simply wear through the tape and then start rubbing paint and eventually even the carbon (not cool on a $4K frame).

GOod luck, 

EM3

 Thanks to everyone who replied.  I will post some photos when I finish the build.