The building code (2003 IBC) has this to say:
"1906.3 Conduits and pipes embedded in concrete.
Conduits, pipes and sleeves of any material not harmful to concrete and within the limitations of ACI 318, Section 6.3, are permitted to be embedded in concrete with approval of the registered design professional."
And, generally the cover requirements are the same as for rebar:
MINIMUM CONCRETE COVER
CONCRETE EXPOSURE MINIMUM COVER
1.Concrete cast against and permanently exposed to earth 3"
2.Concrete exposed to earth or weather
No. 6 through No. 18 bar 2"
No. 5 bar, W31 or D31 wire, and smaller 1Â½"
3.Concrete not exposed to weather or in contact with ground
Slabs, walls, joists:
No. 14 and No. 18 bars 1Â½"
No. 11 bar and smaller Â¾"
Primary reinforcement, ties, stirrups, spirals Shells, folded plate members: 1Â½"
No. 6 bar and larger Â¾"
No. 5 bar, W31 or D31 wire, and smaller Â½"
Engineers and concrete contractors look at embedded conduit as a necessary evil. Conduit doesn't provide much in the way of strength and can often weaken the concrete if not installed correctly. Keep your embedded conduit runs at least 2 inches back (or structural engineer's requirements) from the finish to avoid "pop-outs", and allow space between the conduit and the rebar for the aggragate to flow all around both. If installing in floor slabs, install the conduit under the wire mesh.