I think a backup solution separate from your storage solution is the way to go. The problem with storing your backups on a NAS is that it is likely in the same environment as your existing storage, especially if you're using it for both backups and storage.
Which means if something goes wrong, like power surge, flood, fire, tornado, spilled cup of coffee, insert crisis here.. your data is gone.
Tape is not as "cool" as backup to disk, but it is absolutely the easiest way to get your data off site. Tapes are robust, have relatively easy storage requirements, don't mind bumps and shocks the way a pulled HDD do. A tape solution is also easily expanded as you can always get more media as your data grows. If your data grows beyond what you can get on your SAN solution you need whole new NAS which costs a TON more than does more media.
LTO4 is in fact awesome, but lots of other tape media/software will let you encrypt on your way to tape. Downsides to tape include the need for backup software (however with linux tar is fine) and you have ongoing media costs as well as the need to rotate your media so unless you have a tape changer you need to make sure someone pays attention to the tapes every day.
Things to remember when dealing with tape and moving it off site yourself include:
1. don't put it on a heated car seat
(the tape may get wiped)
2. don't leave them in a very hot car
3. store it in whatever orientation
means the tape is hanging on it's
spindle, if it slumps it may become
unreadable in as little as 18 months
vs many years the other way.
4. make sure more than one person knows
where the tapes are off site and how
to access them.
5. Test restores frequently No less than once a month you should test restore something.
I have used a SAN as backup to disk target with NetBackup in the past. It works pretty well, and the restores are fast and easy.
In short I'd definitely split the solution.