Q: Why don't sites I"m a member of show up in the Memberships Web part?
I've run into this several times when doing POCs with customers. The customer wants to see all the cool MySite stuff and so you add a user to the members group, log in to the site as the user, then browse to the user's MySite to show the Memberships Web Part, and ....., the site is not listed in the Memberships Web part. Well, normally I"ll do an incremental crawl and show how the site now magically appears. I assume a full crawl will do it as well but I'll leave it to you to verify. Some really smart folks tell me that I can also do the following:
stsadm -o sync
Check it out and tell me if it works...