i don't know if multiple logons would be considered a flaw by reeltime or not. but, i think you should shoot them an email to tell them about it, incase they are unaware.
Nice find :) Maybe they have a limit on how many streams you can have at once - I can imagine a family having a membership, kids want to watch a movie in their rooms, parents want to watch another...
But certainly raise the issue with them! Haha be careful what you ask for ;)
I dont think its a security flaw. It would make sense that they would allow several different computers access using the same account, most families have more than one family member/ and many have multiple comupters.
I would start to get concerned if you could do it with like 5 or more.