I'm also in a market for ereader to read programming books. I'm thinking about 650 or 950 but I have the similar concerns and couldn't phrase them any better.
I see that you've listed PRS-650 as you device. Have you bought it? What do you think?
What is you experience with reading on LCD? As a developer I spent 8-.. hours on front of monitor. I don't mind reading news, blogs etc but I can't stand reading books ... my eyes feel tired ... plus I feel all those things that you've raised in your concerns/questions (no visual gauge, can't glance at the peripheral text, harder to re-read/reinforce/associate nearby concepts etc)