Sounds to me like a change in the anti-aliasing algorithm somewhere between iOS 4.2 and 4.3. If so, you wouldn't see a difference in text rendering between old and new iPads running iOS 4.3, because it's a software change. If you compared iPad 1 with iOS 4.2 and iPad 2 with iOS 4.3, you'd see it. Assuming of course people aren't simply seeing things.
The reason why it's more noticable in portrait mode is as someone suggested; if you double-tap a paragraph in both modes, it's going to be smaller in portrait mode. Smaller text equals less pixels per character, which makes bad anti-aliasing all the more evident. You'd get the same problem in landscape mode, too, if you managed to scale the text down to equal size. Conversely zooming in in portrait mode would make the text appear sharper as well.
That said, a Google image search of "iPad safari" - which at this point brings up predominantly pre-iOS 4.3 Safari screenshots - tells me that the AA was "pretty bad" in earlier iOSes, too, with small text. Comparing them to the portrait 4.3 screenshot posted earlier in this thread, I see the same fuzzy AA in both.
For your viewing pleasure:
iOS 4.2 Safari in portrait (large text):
http://www.tipb.com/images/stories/2010/09/ios_42_ipad_safari_find_results.png
iOS 4.2 Safari in landscape (large text):
http://www.tipb.com/images/stories/2010/09/ios_42_ipad_safari_print.png
Unknown iOS Safari (article is from July 2010, so not 4.3) in portrait (small text):
http://www.ipatrix.com/wp-content/uploads/16-Safari.png
I tried to find a good example of poor AA in landscape mode, but couldn't find any blatantly obvious shots of it - for exactly the reason that text is rarely so small in landscape mode that AA becomes a problem. Comparing that last linked image to the one posted in this thread (from iOS 4.3 on an iPad 2) however, I think they both look bad. No appreciable difference that I can see.
Out of curiosity, do you guys see any difference in how these forums look on your iPads when you use the "smaller font" skin, vs. "forum default"? The "smaller font" skin should look worse, if there's any visible difference.