The piece over at Energy Bulletin is incoherent - he's quoted as saying it peaked in 2004, but he's also quoted as saying there's small net increases anticipated in the next few years. I don't know a time series off hand that shows a 2004 peak - anyone?
I guess it all lies in what his assumptions of "conventional" are. No high sulphur, no oil sands, etc?
There's been discussion that light sweet crude (world production) peaked in 2004 but I can't substantiate it. From this Nymex info
Deliverable Grades
Specific domestic crudes with 0.42% sulfur by weight [sweet] or less, not less than 37° API gravity nor more than 42° API gravity [light]. The following domestic crude streams are deliverable: West Texas Intermediate, Low Sweet Mix, New Mexican Sweet, North Texas Sweet, Oklahoma Sweet, South Texas Sweet.

Specific foreign crudes of not less than 34° API nor more than 42° API. The following foreign streams are deliverable: U.K. Brent and Forties, for which the seller shall receive a 30 cent per barrel discount below the final settlement price; Norwegian Oseberg Blend is delivered at a 55¢-per-barrel discount; Nigerian Bonny Light, Qua Iboe, and Colombian Cusiana are delivered at 15¢ premiums.
I am making an extra effort in my posts/comments lately to make sure terms get defined.
I had a post on that back when the claim was first made.
Thanks, I'll take another look at it.

Can't remember them all....
I asked this once before and got no answer. From what I can gather, the mean density of the worlds oil production is rising  and it is becoming more sulphurous. Various comments have implied that more energy absorbing cracking is needed for heavier grades to match the distribution of refinery products to market demand. Sulphur removal is also, I believe, energy absorbing. This will put a downslope on the curve of consumable products relative to the production curve.

Is anybody willing the to give a guess (or better) as to whether this effect is of significant magnitude to edge the peak of usable products a bit earlier than peak oil production?

This is an interesting twist.
Peak Oil is often discussed as a volumetric quantity.
Maybe we should be more focused on "Peak EROEI" for that determines the net work we can do
If your presumptions above are true, we must reach "Peak EROEI" before we reach Peak Oil.
Sorry, but I forgot to add:
It would be interesting to calculate EROEI at Peak Oil and then project at what point along the downslope tail that EROEI becomes one.
Perhaps we won't get a Gaussian curve afterall. Perhaps the tail will abruptly stop at some date: ER = EI.
Skylar
this not only seems right to me but it links ecology and economics. In Economics 101 the competitive firm can only operate in a lens shaped region where marginal benefit exceeds marginal cost. Due to diminishing returns the cost curve always rises more steeply than marginal revenue at some point at which there is no additional net benefit. This assumes the revenue flow is intact. In predator ecology we require energy value of food intake (ER) to exceed the hunting metabolic rate (EI), which of course gets tougher as the number of predators increases. For whatever reason the dinosaurs couldn't get enough calories to sustain themselves, though a few raccoon-like critters made it. If we don't find new fuel or learn to use less we're headed the same way.
I assume this is from the article you are referring to:

He added that conventional oil production around the world apparently peaked in 2004.
Rubin expects 3.6 million barrels of new oil to come on stream in 2006, but 2.2 million barrels will go to replace declining reserves elsewhere, leaving just 1.4 million barrels of new oil.
He expects 1.5 million barrels of new oil in 2006 and 2007, but less than a million barrels a day in 2008.
Energy companies are finding new oil, but most of it will come from non-conventional sources. Ocean oil rigs are the primary source of new oil today, with Alberta's oil sands tomorrow, with expansion projects rivaling those of Saudi Arabia.

I don't see what is inconsistent. He states that conventional oil has peaked but non-conventional oil will cover the decline for a few more years.
ASPO of Ireland in its newsletter of changed its prediction to conventional oil peaking in 2004 and total liquids peaking in 2010.
http://www.peakoil.ie/downloads/newsletters/newsletter56_200508.pdf