Upgrading from a iPhone 3GS. What should I know?

Welcome to the forum.

So the phone is awesome or we wouldn't be around supporting as much as we do here in the forum. Alright let's get down to the questions:

In terms of plans, it sounds like you are coming from a "family" style plan since you mentioned shared minutes.  Is this correct?  If upgrading there may be some sticking points depending on what line is the primary line and such.  You will need to consult ATT regarding your upgrade options if your line is not the primary; if you are the primary it should be no problem to keep the same plan as you have when upgrading - including your unlimited data plan.  You should be grandfathered in under the old plan and not be forced to downgrade to the 200mb/2gb limits.  If the sales assoc tries to do so, tell'em you'll take your business elsewhere if that's the case and they should back down.  It's not like you're upgrading from a standard phone to a smartphone.

As for getting the best deal for the phone, people have been throwing out different prices for it with the top three being amazon.com, letstalk.com, and wirefly.com.  Each is a little different and you'll just have to read the fine print to find out what will work best for you. BUT make sure you read the fine print!!!

Can't comment on the IRC, I'm sure someone else will chime in on that soon.

As for the apps, yes the android market is smaller than the app store for the iphone but it's growing by leaps and bounds daily because android is open-source and developers are chomping at the bit to put out quality apps and corner you're business.  Yes, you might pay more for pay apps in the market, but for every pay one you'll find 3 more free ones that function just as well (if not better sometimes).

Froyo (unofficial official release) is out there now (there's a couple of threads here about it already) and you can upgrade as soon as you feel comfortable working with android.

If you've jailbroken the iphone and are acomfortable working down in the dirt with your phone you'll find the Captivate to fully envelope your every desire and you'll never look back.  If you are someone who doesn't want to think about what you're phone CAN do, just that it DOES what the programmers originally wanted it to do you'll be better off with the iphone, IMHO.

Drawbacks - no flash for camera, no forward facing camera...umm, that's all i can think of...

I came from an iPhone 3g two weeks ago.  Both my brother and my gf have the iPhone 4, and until 3 weeks ago I thought I would be getting the iPhone 4 as well.
 I am and have been on a family plan with 2 other people (but now 1 as of a month or so ago) and we still had our unlimited data.  But my brother and I downgraded to the $15 package.  So be sure to keep yours if you want to!

I'm not too familiar with rooting/upgrading as I am still stock pretty much.  But after official Froyo I plan on rooting.

A lot of your concerns are about your plan and data, yes? That shouldn't really be changed especially on a family plan.

And the front facing camera...my gf has only used it when my brother wanted to show it off to some friends, they both say that although it is a BIG feature (for advertising the phone) they barely use it.

Hope that helps you in your final decision (even though that was pretty much one sided..but look where you are )

Yeah I would use a camera if you could Skype with it (which eventually will happen).

I agree with the attachable camera part!
My first phone didn't have a camera on it but came with one (as bad as it was) that connected to the bottom.