To your point Warren while my LT heartrate has pretty much remained constant I have experienced an increase in my ability to generate power at LT. A little background. I raced for 10 years (in NorCal) from 1987-95, took a 9 year break, and returned to training/racing a year ago. I've run the 2x20m TT test (every 3 months over the past year) to arrive at/confirm my LT heartrate and have observed my max HR on more than one occassion on our Sunday Ride (our local weekly world championship) so I can pretty much confirm 180 (+/- 1bpm).
I've been following a self-administered program using Friel's periodization program (based on 450 hours/yr) and CTS workouts (Tempo, Steady State, Power Intervals etc.) over the past year. I have noticed that while doing 20-30-40 minute Tempo workouts hasn't raised my LT heartrate, it has raised the work I can produce at LT heartrate (measured on a Cycleops Trainer).
When I first started Tempo workouts I could produce 220 Watts at my Tempo Heartrate, or 88-93% of LT heartrate (again 167 w a max of 180). Now I'm up to 235-240 Watts, a huge improvement for someone who is gentically a Cat 4 (hey, someone has to be pack fodder). But what's really interesting is I've also noticed I can hang with our Sunday group on the climb we do (when things get REALLY intense) , whereas in the past I was getting shelled on the gradual uphill lead-in into the climb.
Not sure what this adds to the discussion but it does support your point Warren that one can increase workload at LT without seeing an increase in LT.
Signed
Seeing Improvement in Santa Barbara