I've skiied my whole life, since I was 2. I just learned to tele last year, because I'd always wondered what those old farts were so proud of.. The 3 years before that I rode an AT setup, the year before that was downhill bindings with alpine trekkers for touring. Before that I would bootpack with a board everywhere I went in the backcountry, snowshoes if I had them. I've done it all when it comes to snow riding - and I can honestly say that I have had the most fun on my tele setup, hands(knees?) down. I learned last year on a pair of fatty icelantic shamans with hammerheads - this year i got a pair of O1's that I'm putting onto my light mountaineering skis in place of the old Naxos. Yep I'm not ashamed to say that I have become one of those proud old farts.., from now on it's all tele all the time for this hipster ;)
Telemark might not 'stress' the knee laterally as much as Alpine but your knees will do WAY more work over the course of the day leading to increases in platellar tendonitus (sp?) as well as wearing out your the cartilage in your knees faster. So while the ACL/MCL issues might be less you are FAR more prone to wearing out your knees with tele.
I'd have to disagree here. The "work" itself is not going to cause problems. In fact, the extra work helps keep those stabilizing muscles stronger. And, as someone else pointed out, it gives you license to drink more beer at the end of the day.
What generally damages cartilage is repetitive impact which is basically non-existent in tele skiing. I've had three knee surgeries due to cartilage damage over the years -- including a micro-fracture procedure -- and none of it was due to tele skiing. My knees never hurt after a hard day on the hills. The quads on the other hand...