Firstly, I'll confess that I don't wrangle design software for a living (I have a programming background)... but I have owned Macs since 1992, and I've owned and used Photoshop since version 3 (that's version 3 in 1994, not CS3!). I've also used Dreamweaver and Flash on a range of machines.
In my experience, what Dreamweaver and Flash need is lots of screen real estate.
Photoshop needs processor speed, memory and fast disk access.
In the last 5 years, increases in processor speed on laptops has more than outstripped the processing requirements of Photoshop. Additionally, now that we have SSD drives, it's no longer necessary to use fast desktop drives/striped disks to make Photoshop really sing.
If you pay for a desktop (and I'm thinking specifically of the Mac Pro), you're paying a lot for it's ability to run lots of CPU cores - and Photoshop is poor at multi-core processing. The straight-line speed of Apple's consumer machines is often faster than the Mac Pro Xeon chips... so the Pro desktop gives you little (if any speed advantage).
So, in summary I'd say that while 5 years ago a good desktop would have blown away a laptop when running Photoshop, nowadays there's really little reason to choose the desktop unless you need to house lots of drives.
Personally, when I upgrade my current machine (a 2008 8 core Mac pro), I'm considering 'downsizing' to a 15 inch MBP - which I think will be plenty fast enough.