In the SR22 you simply put the DME station on GPS2 and you have the distance on the PFD, right in front of you, even nicer than the DME. The difference because of no slant range is unimportant in the real world.
I have DME and ADF on the G1000 and while I probably wouldn't have paid for ADF, both it and DME are handy in Europe as so many approach plates are easier if you have it. Of course you can substitute but the ability to use the ADF for a station passage and the DME plus a GPS range for two separate distances has come in handy more than once. I do always laugh however at using an ADF on the accurate digital G1000 display....
In the SR22 you simply put the DME station on GPS2 and you have the distance on the PFD, right in front of you, even nicer than the DME.
I am curious how you do that on the G1000, I am not aware of a separate GPS2 waypoint you can set.
Slant range difference is not the issue - at 3 degree glideslope this is indeed irrelevant. The issue is that (1) not all approaches have a waypoint that matches the DME zero range and (2) this is not the waypoint that will be the active waypoint until very late in the approach, unless you force it.
The best substitute is the threshold waypoint (RWxx), which usually is right at the threshold or around there. If you load that approach you will have it in the flight plan, but will not see the distance displayed before you get to the FAF. And even then there might be a waypoint between the FAF and the threshold. And your ILS chart will NOT tell you what they are, although you can usually correlate them to things that are charted.
There are also (rare) approaches with non-coupled off-field DMEs, approaches where your missed approach starts half a mile above the DME level so the slant range difference DOES matter, etc. etc.
All of the above is fraught with errors. All fine if you are familiar with the approach or want to spend the time pre-flight to figure this out, but not something to make up.
Or, of course, you could just believe the ILS glidepath and your QNH setting and forget all the altitude/distance checks. Prob 99.9% this will be fine, the one time you have mis-set the QNH by 10 hPa and are unknowingly 300ft too low could kill you.
The difference because of no slant range is unimportant in the real world
as long as you have radar coverage. Without radar applying conventional longitudinal separation ATC needs 10DME spacing for instance to descend another traffic through your flight level. At FL360 your DME will show 6 DME at DME station passage (6000ft~1NM). A GPS will show 0 DME. In certain cases ATC could think 10NM separation were given when there are only 4 NM. However I can't say whether ATC would use DME separation in the vicinity of DME stations where slant range is a factor ( in case of radar outage or non availability, see Africa )
Oh, come on! We are not talking about FL 360 here but about the need for a DME in a typical GA scenario.
In a typical IFR approach it does not matter one bit if you use a DME distance or GPS
CObalt,
I fly a Avidyne equipped SR22, so I don't know.
With the Entegra PFD you use the "AUX" line selector key and select the sourse "GPS2". On the second GPS (GNS430) you can do a direct to any DME station. But you have to switch off "Auto crossfill" from GPS1 to GS2 because trasnferring the flightplan frm GPS 1 to 2 will overwritethe course to the DME station.
So you fly your route /approach with GPS1 and have the DME station on AUX. It's pretty comfortable and puts the distance information right on the PFD!
The GPS cannot be used in place of DME unless the GPS waypoint corresponds to the DME=0 location, and that location may not even physically exist if the DME is set to read a different value from the actual, as is sometimes the case on approaches.
This is a big gotcha.
Typically, a DCT to some airport gives you the GPS distance to the centre of the aerodrome, which might be the middle of the instrument runway, or maybe not.
Whereas a DME gives you a reading to a specific defined point.
The two are not interchangeable, other than by accident. The difference can be pretty big.
The procedural separation for enroute IFR traffic is much greater than some small error.
Cobalt, of course a DE would be even safer - but if I have the ILS Approach active on the DME together wih the vertical profile and do an altitude check at FAF, OM.... it should be very safe.
So you fly your route /approach with GPS1 and have the DME station on AUX. It's pretty comfortable and puts the distance information right on the PFD!
But that is the GPS distance to a navaid and not the DME distance. Not only slant but offset is not taken into account.
Oh, come on! We are not talking about FL 360 here
once you'll upgrade to a Piaggio Avanti it will be in reach 
No, you are right. Flying approaches it doesn't matter. (waypoints corresponding)
