Can you do Pull Ups using the TRX?

Absolutely! If you do not have a place where you can attach the TRX high enough to perform pull ups with your legs straight simply start from your knees and perform pull ups. For an even more challenging pull up start from a seated position and pull up holding legs in front of you! Suspended pull ups are a great challenge because you have to control your center of gravity and stabilize much more. The rotation of the handles provides all kinds of variations of hand position which will change the exercise. 

What about a hinged-hip pull-up?  So, feet on the ground, legs straight, bent at the hips toward the anchor point, crown of the head pointing at the anchor, pulling straight down?  It's not quite the same motion or load, but it'd be a good substitute.

6 months ago I was unable to do any pull ups. After two months of doing 15 assisted pull-ups alternating hand positions and using the TRX I was able to do three in a row un-assisted. I am a 6'6" 210lb guys so that's a lot of wieght to pull up off the ground.  

I was able to shorten up my TRX and do a combo of a row then pull up.  It was way killer and still paying the price today. 

 At the lower end you can use the foot cradles to assist a regular bar pull up

Certainly the rows will help to gain the strength to weight ratio to a point where pull ups are more feasable

Try overshortening the TRX so that you are able to sit underneath the handles and pull up from here. Once you are able to do this you will be able to progress back to the bar to do full pullups

You can also check out the link below to show you how to set up the TRX in dual point mode so that you can use it for traditional pull ups, dips and muscle ups

Dual Point Anchoring

Fraser's link seems to work now. 

Here is the link again though. You should be able to copy and paste or click on Fraser's link below to view the article.

http://community.fitnessanywhere.com/blogs/mfp_interviews/archive/2008/09/29/configuring-the-trx-with-dual-point-anchoring.aspx