The fact that Tradehill moved the ⓑ from the original address to two other addresses should be considered a positive sign rather than a negative one (now that they've responded to your concern). This suggests that they have some internal separation between private keys, which might be able to limit the damage in case of accident or malice in which one of the private keys is stolen or destroyed. Or maybe they don't have any effective firewalls internally between private keys, but they might.
In contrast Mtgox is obviously not doing that, since they are using a private key which currently controls almost half a million ⓑ to sign small transfers:
https://bitcointalk.org/index.php?topic=53848.0;allThat just seems unnecessarily risky to me.