Good point on the Dual Core CPU's causing half the notebook line to be released at a later date.
Another factor is the the MBP got speed bumped in the Fall. Hence they could take a longer delay.
And if it has to be staggered then it should go:
Round 1: Mac Pro, iMac, 15" / 17" MBP
Round 2: 13" MBP, 13"/11" MBA (with possibly a redesign?)
Round 3: Mac Mini
The problem is that Apple has largely done single models per month for last several years. There have been some double-ups . In 2010 there was a iMac launch alongside a Mac Pro "announce for next month". In 2011, the Mini and MBA doubled-up after Lion (10.7) launch.
More likely would be something like this.
1: Mac Pro ( perhaps in April 24th - May 1st timeframe ** )
2: iMac ( late May. 5 Tuesdays in May so can stagger by 4 weeks )
3: MBA ( demo only at WWDC *** )
4: MBP ( July after/with Mountain Lion. defacto double-up shipping with MBA )
5: Mini ( August due to logjam in July )
** If waiting on Nvidia's newest cards that should uncork by May 1st. Similarly, the lower end E5 1600 should get freed up by 32nm Core i winding way down. At very least, they should have enough insight to announce at that point and ship later in month (like some others are doing now).
*** If Apple wants to make a major demo feature of HiRes Displays then MBA is most likely candidate. It has smaller screens so the model most likely to go "all HiRes" (it is already a bit ahead of Mac line up in that respect. HiRes would get rid of those screens making fonts smaller than they should be).
The iMac and external displays are too large for HiRes without blowing up the pricing. The latter likely means Mac Pro and Mini aren't good options. [ Mac Pro could if Apple jumped back into the pure display market, but I doubt they will do that. ] The MBP 17" is also a bit large to crank the resolution up on without price blow up. So MBA is more likely candidate.
If Apple doesn't need a WWDC demo then the Mini could possibly be pushed into the "early in June as possible" slot if missing "back to school" sales (or some major mini purchasing cycle ) were going to be an issue. Otherwise, June goes "dark" like 2011 for similar reasons.