In the course of that search, I just came across a terrific comment by Eddie Campbell from a few months back:
The 'thought' bubble (or balloon) is one of the few inventions truly indigenous to the twentieth century comic strip and it would be sad to reject it in order to make comics more like movies (see comments on this theme under 'Things' two days back), or because it is somehow pictorially unseemly for a tough heroic figure to have fluffy clouds around his head. When you tell an anecdote orally it's commonplace to say 'I thought' and 'she thought' etc., and perfectly logical to codify that on paper in a thought bubble. And if it makes your character less heroic, try taking the pole out of his ass.
I've always liked Eddie Campbell's comics work well enough...but I absolutely love him as a comics historian and theoretician. And this point in particular is something comics writers argue about a lot. I think Campbell's got it exactly right.
Panel 1. RAB sits at his computer, frowning in thought, his chin resting in one hand.
RAB (thought): Maybe the chronic inferiority complex of comics creators is to blame for this.
RAB (thought) 2: It's like a child trying to look adult by eliminating anything that seems childish.
Panel 2. Looking up from street level as the anarchist character V leaps from a rooftop in a scene from V FOR VENDETTA as drawn by David Lloyd. Something like this image.
CAPTION: Alan Moore and Frank Miller made comics look "cool" and "adult" by not using thought balloons. Or sound effects.
CAPTION 2: They made comics look like movies -- like "grown-up" entertainment.
Panel 3. A shot of the Marvel S/M fetish character PENANCE with his fists clenched in a moment of supreme angst.
CAPTION: It worked for them as individuals...but in general, what could be more childish than trying to look all "serious" and "adult"?
CAPTION 2: You just end up looking silly.
Panel 4. RAB gets up from his desk, leaving the computer behind.
RAB (thought): Most old people would give anything to be young again.
RAB (thought) 2: Real maturity goes beyond that, to an appreciation of good things from both childhood and adulthood.
Panel 5. RAB sitting in the living room, in front of a television set. In one hand, he has a remote control pointed at the screen to turn the tv on.
RAB (thought): Comic books don't have motion, they don't have music, they don't have the sound of human voices. They're not movies or television.
Panel 6. On the tv screen, we see KIM POSSIBLE talking into her Kimmunicator.
RAB (thought, off-panel): With all these limitations, it's stupid to throw away some of the few tricks we have out of some misplaced desire to look more "grown up"!
KIM (voice from television): So what's the sitch?