Right so after you log in with your phone you sync to the site. Log in to the site and you will see a list of apps installed on your phone.
Me for example I just got rid of my droid for the X. So I just installed Appbrain logged then went to my office and appbrain.com and was able to see the list of all the applications that I had on the old droid. I then copied that checked those apps and selected add to X which was on the drop down menu. Let me know if you need a screen shot.