I know there's some notes on that topic in the Monster Manual, and Savage Species has some stuff on ECL which might also prove useful. I don't know about any other sources.
An article of this nature really would be helpful. One thing that I think needs some serious consideration for 4th edition is standardization of the challenge rating system. For example, a 5th-level NPC is NEVER as challenging as a CR 5 monster unless you load him up with powerful magic items and give him max hp rolls. Besides that there are a number of creatures with wildly diverse CRs that are of the same approximate difficulty. Most of this is contained within splatbooks. For example the average difficulty of a CR 10 monster in the MM3 is more overpowering than the average difficulty of a CR 10 monster in the MM. Perhaps if we saw an article like this in Dragon it could wind up serving as a prototype for 4th edition CR calculations.
Its for the very reason that Challenge Rating for monsters as they are listed in the Monsters Manual and other places that I feel that Challenge Ratings are bogus and a complete waste of time in trying to figure out. Personally I think that challenge ratings should be completely thrown out. Base it on what you think the party should be able to handle. I've found more often than not I have to find monsters than are a good 4 or 5 points higher on the monsters CR to even make it a challenge for most of my players that have researched through a good number of books to find the rights feats to make their characters the way they want to.
I would really like to know how to calculate Challenge Ratings for if/when I create a new creature to submit. I have to admit as well that some NPC profiles do not seem to be well balanced. A single NPC with a ECL of 5 is given a CR of 5. So that means that a 5th-level NPC wizard is meant to be a balanced threat for a party of four 5th-level PCs? If the party includes a 5th-level wizard of its own, then there must really be a problem with how Challenge Ratings are calculated. I hope Dragon does an article to deal with this and help clarify things soon. Or maybe Paizo or Wizards might be willing to even do it as a web article sometime in the near furute?
thre is no sure method of calculation when it comes to a creatures CR, at best any sort of CR "chart" weould give you a gross guesstimation of a creatures CR which would then have to be tweeked through play testing (This is the method I use, my guesstimator" is fairly close to the one in the 3.0 hoiw to create a monster article by Skip Williams in dragon #276) it is fairly accurate, seldom more then 1-2 CR's off for most creatures, however it can be way off when dealing with creatures with lots of HP and few or no special abilities, or the reverse (very few HP and lots and lots of SA).
A single NPC with a ECL of 5 is given a CR of 5. So that means that a 5th-level NPC wizard is meant to be a balanced threat for a party of four 5th-level PCs? If the party includes a 5th-level wizard of its own, then there must really be a problem with how Challenge Ratings are calculated.
Well, two things. First (and probably most important) ECL doesn't always equal CR. This is especially true if there's a level adjustment involved. Let's use your wizard as an example. A human 5th level wizard has a CR of 5. Now, take that same wizard and add the half-celestial template. If that were a PC, the ECL would be 9 - the template carries a level adjustment of +4. However, the CR is only 6 - when the template is applied to a creature with 5 or fewer hit dice, it adds one to the CR.
OK, so on to the second comment. A 5th level elven wizard is in fact a CR 5. This means the wizard is expected to require the PCs to burn through about 20% of their daily resources before they defeat the wizard. Having a CR equal to the average party level doesn't mean it should kick the PC's collective asses. It simply means it should offer a fair challenge and suck up their precious resources - hit points, spell slots, potions, arrows, whatever.