This morning when I saw Chandra's post, I also did a search across the novels. I became interested in Austen's use of shy vs reserve earlier in the summer when I was checking the text for something in Persuasion and came across the quote of Benwick being shy but not reserved. It stood out like a beacon as I thought of all those debates on P&P when Darcy has been referred to as shy. We know though, that Darcy is described as reserved while Georgiana is the shy one and with these examples from Persuasion and P&P, I came to the same conclusion that you did---that Austen clearly perceives the terms of shyness and reservedness as two distinct behaviors.

I also looked up the terms in Johnson's:
Some definitions for reserve are 'something concealed in the mind, not frank, sullen, not open'
some of the definitions for shy are: 'cautious, wary, not familiar, keeping a distance, reserve'

As I was mulling over Chandra's question of why Marianne thought reserve was worse than shyness, I reasoned that shyness appeared to be looked upon as a natural 'affliction'. As a matter of fact, at one point in MP, I discovered that Austen actually refers to the term of natural shyness. Also shyness, on at least two occasions (in MP and S&S), was linked to awkwardness and lack of confidence as we see in regards to Edward. On the other hand, being reserved seemed to have been a choice and therefore censured as a sign of purposeful concealment and/or being unsociable. It is generally included within a list of such traits as haughty, proud, and disagreeable, etc. After the search, among my notes was Elinor's observation of Col. Brandon whom she was willing to make an exception for when she perceived him to be reserved by deciding she liked him anyway. Now, I have to ask why the Colonel is so reserved? In any case, it does support the fact that reserve had a negative connotation if it was something that Elinor chose to overlook.

I agree with your and Barbara's thoughts that Marianne did not likely intend to imply the word to mean that Edward was concealing something but he took it that way because, as we now know, he does have a secret and was worried for a moment that he had been found out and Marianne was referring to the secret engagement.

I have to thank you Rachel because you saved me from having to type all those quotes with the attending comments. :) And isn't it amazing the way knowledge of the specific shades of meaning in Austen's works can make such a difference in interpretation and point to the foreshadowing of events?