Can a GPS system be used to identify fixes on conventional instrument approaches? Is the use of a certified IFR GPS receiver approved for identifying fixes defined by VOR radials, on a conventional (non RNAV) instrument approach?
For example, in KCDW LOC RWY 22, the intersections DOWDY, FAAIR and KOLLI (IF, FAF, step down fix, respectively) are all identified by VOR cross radials (TEB VOR). Can they be identified solely by reference to the GPS database?
Another example would be KABE LOC RWY 13; IZKIP is a step down fix identified by a cross radial from ETX VOR. However, the chart states: 


  IZKIP FIX MINIMUMS (DUAL VOR RECEIVERS REQUIRED)


Can a GPS be used to identify IZKIP in this case?

According to AC 90-108:


  ...operators may use a suitable RNAV system in the following ways... Determine aircraft position relative to or distance from...  a named fix defined by a VOR radial.


Therefore my understanding is that named waypoints/intersections defined by VOR radials on a conventional IAP like in the examples above, can be identified using a GPS, except perhaps when the chart requires dual VOR receivers.
Your reading of AC90-108 is correct. As long as the RNAV equipment is operated and installed in accordance with TSO-C129/-C145/-C146 it can be used as a "suitable RNAV system." (ref: AC90-108, para. 6 a.).

Also, see the AIM, 1-2-3 b. "Types of RNAV Systems that Qualify as a Suitable RNAV System."

Lastly, as far as the requirement for "Dual VOR Receivers" for IZKIP, since you are using a "suitable" (and legal) alternative method by using the GPS database fix location, no additional restriction should apply. Also, it is my understanding that when the subject stepdown fix (identified by a VOR radial) is inside the FAF, as is the case with IZKIP, the dual VOR requirement is for the purpose of presetting the missed approach holding fix.  Again, not an issue if you are using a "suitable RNAV" system for identifying the fixes.