Firstly, Time Machine isn't your best option here, since it will happily nuke older items when it runs out of space without any warning or dialog. And, since you're talking about photos here, I think what you're really looking for is an archival solution (not a backup solution).
I use a Drobo for local archives and I use JungleDisk for offsite archives.
The Drobo would already deal with all the mirroring for you and what I love about it is that when you get low on space, you simply add a new drive from virtually any manufacturer in an empty bay, or replace an older disk with a larger replacement. LOTs of headaches erased here.
I do use my Drobo for a TimeMachine backup too, so it will serve your backup needs too.
JungleDisk is an nifty offsite mirroring tool that uses Amazon S3 for the storage layer (it can also use RackSpace, since they purchased JungleDisk awhile back). This is a "set-it and forget it" thing that backups up the changes off my local archive at regular intervals.
So my strategy is that I keep my photos nicely organised on my Drobo, and that is silently backed up offsite by JungleDisk. I can easily access all my photos locally as required. I use Aperture and I configure it to use unmanaged masters so that they aren't stored inside of an Aperture Vault–only the meta data is. This allows atomic file changes to be efficiently managed by JungleDisk.
I don't think Lightroom does photo management, so you shouldn't need to bother with that.