[aristeides]

I believe the one amendment that has been passed by state conventions was the 21st Amendment (repealing Prohibition). Conventions were chosen in that case, I believe I have read, because the amendment could thereby be ratified more quickly. And I think the 21st Amendment was in fact the amendment ratified most quickly, within a year.

[AntiGuv]

I just tracked down the following link, if you're interested, which explains how the ratification convention process works (you're quite correct about the 21st):

Constitutional Topic: Ratification Conventions

I was wondering how it all works; looks like each State has its unique guidelines. Could be exciting if this makes it out of the Congress specifying the convention process of ratification!