I recorded my acoustic piano on a song recently and, while the piano is in tune with itself, it's slightly flat from concert pitch. What's the best way to pitch the track up a bit to be in tune with the rest of the instruments? I already tried using the default Cakewalk CFX Pitch Shifter, but heard way too many artifacts, so I cancelled that.
 

If you have the Melodyne (Editor) in your tool kit you should be able to correct pitch easily, efficiently and without artifacts.  The Single Note version that comes with X3 would not be of much help unless your piano passages are truly "single note" (with no harmonic chord structure within the performance).

The old school method of pitch adjustment is to change the speed of tape playback. In other words, record it on tape and then speed up the tape motor until it's in tune. You can figure out just exactly how to re-tune by recording a single note and then playing it back and slowly speeding it up until the tuner says the note is in tune.
 
There are tools in the digital world to do something similar. In a sense you are then doing two steps at once...
1. Speeding up playback
2. Time stretching to keep playback duration the same.
 
As you may know when you set up a track for good time stretching, marking transients can really help the algs that do the time stretching.
 
Interestingly, there is a RME tool for pitch adjusting through the UCX interface I have. I can detune up or down maybe a 1/4 step.
 
I haven't really helped you, but it's the best I've got.
 

Have you tried this?
 
http://www.cakewalk.com/Support/Knowledge-Base/2007012960/How-do-I-change-the-Pitch-or-Tempo-of-Audio
 

Use Sonar's "hidden" variable speed capabilities, which I've discussed at length in this forum. It functions exactly like using tape varispeed - no artifacts, no fidelity issues at all. The downside is that like tape, it changes the duration. If you want to change pitch while preserving duration, that's a much more difficult task.

I found the instructions. BTW my next Sound on Sound column has details on different varispeed effects in Sonar.
 
Like true analog  tape variable speed, speeding up shortens duration and raises pitch, while slowing down lengthens duration and lowers pitch. The reason I'm doing the "precision" solution first is that the most common variable speed tape effect I did by far was speeding up the final mix by 1 or 2%, which of course also raised pitch by 1 or 2%. This technique easily accomplishes this function, as well as the "Chipmunk effect" and a bunch of other cool stuff. (I also have a solution for the "overdubbing the slightly out of tune instrument" problem. All of these techniques are improvements on the suggested workarounds I gave in the other thread.)
 
The speed change is up to four times faster or slower, and you will not hear digital artifacts. In other words, it’s just like using tape with the only limitation being that this particular solution trades off precision for not being continuously variable. Ready?
 
1. Open the clip in the Loop Construction window. Note that this IS NOT being used to create a loop or groove clip and the only thing they have in common is the name of the window.
2. From the Clip drop-down menu, enable Stretch On/Off.
3. Move the Threshold slider all the way to the left (0%) so all the markers disappear. This is very important.
4. The two right-most fields adjust semitones and cents respectively. Do not enable the Pitch button! That will just confuse things. Cents will adjust +/-49 cents which should be enough. If not, for example if you need to make the pitch 70 cents sharp, set semitones to 1 and cents to -30. (For Chipmunk effects, set semitones to +12 .)
5. Render the clip, and it will reflect the pitch/speed changes you made.

Craig always has the tricks!