I have a friend who wants to bike from Pflugerville (say the corner of
Heatherwilde and the Pflugerville Loop) to the Whole Foods in Gateway
Plaza. I've done this ride once or twice and can offer suggestions, but
maybe someone who lives in the area has some specific suggestions for
the best way to go without getting run over?

As far as riding to The Domain goes, there really are no great routes
connecting The Domain or Gateway areas. If you are coming from the
south, riding north east of Metric might get you to Gracyfarms where
you can turn west to Gault which enters the Domain on its north-east
side. If coming from the north, you have to ride on Burnet for a
little bit. Once in the Domain the speed limit is 25 so sharing the
road or taking a lane is no problem.

Following is actually from Austin's Bicycle map.
I've taken the roads west of I35 some and all seem fine to me. Not sure about Cameron, but it is a short section.

45th East to Red River.
North on Red River to 51st
East on 51st to Berkman
North on Berkman to Coronado Hills
West on Coronado Hills to Cameron
North on Cameron to Rutherford.

I'll also recommend avoding the worst of the Ohlen/183 intersection. Heading north, you can take a right on Contour to hook up with Fairfield, cross 183 on Fairfield, and hook up with Parkfield that way.

The reason to do this is that, when heading north on Ohlen, the driveway into Target just past 183 provides a very high risk of getting hit with the right turn of doom.

I too use Fairfield for crossing 183. One detail about my route is that I take my second left off of Fairfield to head up to Ohlen. By doing this I can make an easier "jog left" crossing of Ohlen (turn left onto Ohlen then turn right off of it). If I make my first left off of Fairfield, I must make my left turn off of Ohlen, meaning a possible wait in the left lane and a turn across oncoming traffic.

After crossing Ohlen, I take Hunter's Trace up to Colony Creek, and pick up Parkfield there. I admit that I do not like riding on Parkfield northbound (uphill) between Rundburg and Rutland (downhill is no problem), and will often (should time allow) turn left at the stop sign on Quail Meadow, using QM to cross Rund and Rut, then cut back to Parkfield.

Two common destinations for me in this area are Regal Plastics at Metric & Rundburg (for coroplast, Lexan polycarbonate sheets, and reflective vinyl), and T&S Seafood Restaurant (for dim sum) at Ken & Lamar.

Here is an alternative to taking Payton Gin Road (a rather busy and narrow street that is prone to speeding) to Parkfield:

Just before you get to the intersection of Payton Gin Road & Olen (the first traffic light)
On your left is a convenience store
Cut across the parking lot of the convience store and cross Payton Gin Road
There is a neighboorhood street almost directly in front of you.
Take that to the end. It is wide, slow and not busy.
At the end of the road turn right.
Follow this road to Parkfield and turn left.
Sorry I don't know the names of these steets and don't have time to Google it.

This route is shorter and safer. (be careful crossing Payton Gin Road after the convience store)
Parkfield has bike lanes almost the entire way.

I work on the South frontage of mopac just North of
Hyridge. As I'm coming from near Anderson Lane on the
East side of Mopac, I find that I spend 90% of my time
riding sidewalks crossing and along Mopac (facing
traffic, as the frontage has only one side sidewalked)
to get to work. Should I ride on the road? It seems
hair-raising!! Those cars drive 50+ mph. Even
crossing at Anderson or Steck those cars are going
40+. How do you guys keep from getting hit by side
mirrors in this area??