Yeah. Titanium backs up your apps and data to sd card. Which mean you can restore it from ROM to ROM.
Nandroid backs up everything above the radio level. Kernel, boot.img, /system, basically the whole ROM.
So say I was on ROM 1 and wanted to change to ROM 2. I'd do a Titanium, then a Nandroid (or the other way around, whichever's most convenient xD).
After I do my wipe and install ROM 2, I don't have my apps! But if I download Titanium from the market, it'll see my backups on my sd card and I can restore them.
Now say I don't like ROM 2. I can use my nandroid backup to change back to ROM 1, with all of ROM 1 intact.
Hope that helps =]