Aaron,

  The deep n-well that I am used to using is very different from the n-well
used as the body of a p-channel transistor. Please see page 15 of the following
slide set,

http://www.jazzsemi.com/docs/SiRF-06_Presentation.pdf.

A deep n-well is not used as the body of a p-channel transistor, it is used
to isolate the p-well used to create the body of an n-channel transistor
from the substrate. As a result, using a deep n-well makes the n-channel
much less sensitive to noise coupled through the substrate.

These structures are not interchangable, either you use a deep n-well and
the isolation is increased or you do not use a deep n-well and the body of the
n-channel is not isolated from the substrate.

Some additional comments, using a deep n-well improves the isolation of the
n-channel body and substrate. Depending on the frequency and the layout
there may still be isolation issues. That is,
1) There is still capacitive coupling so there may be leakage at high frequency
2) You need to make sure that the deep n-well does not contribute noise, that
   is, the usual design practices to maintain a low-noise substrate: needing a
   clean supply, plenty of substrate contacts[deep n-well contacts], guardbanding,
   etc., apply to deep n-well layout  
3) Whether or not you use a deep n-well in the design, you should still include
   analysis of substrate coupling in the design flow


                                                                      Best Regards,

                                                                         Sheldon