There are no currently known vulnerabilities in encryption in Ubuntu 11.10. There have been some. Generally the ecryptfs vulnerabilities involved an attacker already logged into your system who could cause denial of service. There was a LUKS problem in which users were surprised that a simple configuration option in a partitioning tool could fully and permanently destroy a partition.
In about 5 * 10^9 years, we expect this planet will be engulfed by the expanding Sun. AES-256 encryption can quite possibly resist attack for that long. However, as you seem to be aware, there are many other potential weaknesses and they bear repeating.
Will you know that you encrypted the disk effectively? It's complicated. Thoughtful people disagree about which installation options are sufficiently effective. Did you know that you're supposed to install full-disk encryption from Ubuntu's alternate install CD using LUKS, not ecryptfs? Did you know that LUKS has stored the passphrase in RAM in plain text, or that once a file is unlocked by any user via ecryptfs, then ecryptfs does not protect it from any other user? Was your disk ever connected to a system that permitted attaching unencrypted storage, rather than setting an explicit SELinux policy to prohibit that? Where did you keep your backups of your encrypted disk? You made backups because you knew that encrypted disks are much more sensitive to normal errors, right?
How deeply are you committed to keeping your passphrase secret? What jurisdictions will you be in? Would you be content to go to prison? Are there other people who know the secrets that are protected by your disk encryption? Do you want to pay the price for your secrets even if those people reveal them?