I'm considering getting the new Mac Mini as my main desktop computer (replacing my 2002 which works like a charm, but a bit sluggish in today's world.)

My husband brought up a good question:

If everything (docs, apps, EVERYTHING - a backup of my main desktop Mac) is on an external drive, could it be plugged into the MacBook Pro (newer one, early 2008 model) and apps used normally?

There may be an issue with apps that have to be authorized to a particular computer when registered. However that brings up a second question:

What about doing the actual install of applications to an EXTERNAL drive, then plugging it in to whatever computer desired for work.

I can answer your question and will in just a sec, but the question I have is what are you trying to accomplish by booting a MBP with your existing computer/drive?

Now on to the answer:
The short answer is no. If you were to clone your existing computer to an external drive and then try to boot any newer Mac, the OS from your older computer wouldn't be compatible.
So at a minimum you would need to clone your computer to an external drive, then take the OS discs that come with your new computer and update the OS on the External drive before it would boot a new Mac.

So now we're back to the why? question....

When you get your new Mac mini or MacBook Pro you would be able to use the Migration Assistant (if that's what it's still called) and it would automatically copy over all your apps, prefs and documents onto your new computer. If the apps are compatible, they would function as they always did. Some apps may need to be reinstalled, but that's about it. I've migrated from computer to computer over the years using this method. Each time I say to myself, I should start fresh! Install everything from scratch (which is the better way!), but then I look at all the stuff I have going on that I would have to reconfigure and I chicken out and hit the migrate button. I just never seem to have the down time to start completely over from scratch, installing, customizing, reconfiguring all my apps, websites, settings, etc.

If and when I do upgrade to a new Mac Pro however, I will be configuring that Mac from scratch. My Mac Pro is a production machine primarily used for video editing and page layout. There's not a lot that needs to be configured, so I will be setting that one up from scratch to have a nice clean start.

Thanks Terry. I should have thought of the OS issue.

What am I trying to accomplish?: - Possibly using my MBPro (purchased last fall) for work currently done on the (older) desktop Mac. - Thus saving the purchase cost of a new computer.

Actually, I think I envisioned booting from the MBPro, but accessing just apps & docs from an external drive. Have resisted turning the laptop into a "main computer" for the time being. However, it makes more sense to just go ahead & get the Mini.

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MIGRATION QUESTION:
I would likely migrate the current desktop stuff to a new computer, as you recommend.
1. Does the Migration Assistant allow for moving data from earlier version of the Mac apps (Mail, iPhoto, etc.) to the new up-to-date ones on a new machine?

2. Should apps be de-authorized on the old one, then moved, or re-installed?

1. Yes, Migration Assistant will move data from older apps such as Mail to the newer version on your new computer.

2. It's always better to reinstall from scratch. However, if you de-authorize, migrate and authorize then you should be all set too with most apps. Some may still require reinstalling.