I rode the TLH twice, in September 2009 and August/September 2010. Ride report links are in my sig below.
I would recommend pushing the trip back a couple weeks -- the bug situation is much better in September. On the '09 trip we never even used our bug spray, let alone our headnets, and on this year's trip the bug spray and headnets were only used occasionally.
The Phase III section between HV-GB and Cartwright Junction is finished -- but be aware that the road crews are now hard at work paving Phase I between Lab City and HV-GB. So you will encounter construction zones there, as well as plenty of freshly graded gravel. I know it's a little disappointing that the road is being paved, but think of the paved sections as a chance to get a good look at the landscape around you -- it's pretty phenomenal, but you don't get to see much of it while riding the gravel because you're too busy concentrating on the road.
Little of that work had begun yet when I did the trip in '09, and the road conditions were largely very good (plenty of solid surface to ride on where the gravel had been scraped away). This year's trip was much more difficult because nearly the entire road was loose gravel with few or no tracks in it. So be prepared for wildly variable conditions.
Have a great time!