Not certain what the current situation is, but the spec on paper can be very deceiving.
The prime difference is that laptops have low power versions of similar sounding chips which - surprise surprise - don't perform as well in the real world. Add to that graphics which again are heavily compromised in laptops and, although the manufacturers have been careful to make it sound good, side by side you get better bangs for bucks from a desktop. Fortunately, one of the big bottlenecks of a laptop - disc access times - has gone if you go for SSD. If you don't then disc access is again another laptop downer.
Which is all fine and dandy, but as most people only use their computers as browsers and word processors, in the real world a laptop gives you better value if you are one of those users.
The applications which would show up a laptop include: Graphics applications (photoshop etc), 3-D, Gaming, Database/large spreadsheet, Virtualisation