I know what you mean about the app2sd and that you can't currently be a non-market beta tester (using an Aria). I am not 100% but the email I got back when I asked about sideloading seemed to indicate that when 2.2 was pushed by AT&T then sideloading would be allowed as well. I can of course only pass on the info I get and it isn't always correct. As to the Samsung Captivate as I get info I'll pass it along but I'm not always kept in the loop on stuff.
To s.m.knipe: currently the option to sideload doesn't exist on the Aria. If that option will be on the Captivate I couldn't say, but I wouldn't hold my breath to be honest...we all know AT&T and how they like to control the user experience (one reason them and Apple get along so well).
I'm curious as to what the Samsung UI is like for android devices as I really like the HTC Sense that's on the Aria. In any case I'm just happy to have an android phone that is inexpensive enough for me to get that isn't the backflip.