If there was a poll I'd vote for Carls first suggestion. For some reason it just seems most "OpenACS"-ish to me - it's clear and simple, and somehow harmonizes with the philosophy of OpenACS, although I am afraid I cannot explain why. Maybe it's just because it takes much of the existing design, which I actually like and would find it nice if some pieces of it could be kept. The other designs, although all very beautiful, look very generic to me, e.g. they could as well be for any commercial product (I immediately associated icecream with #1 and #3 😉
Some random comments:
I agree with Carl that it'd be nice to have the top bar served on every page, above the page title and the breadcrumbs, although I find that the one in his suggestion would still be a bit too tall, so I'd vote for the suggested one on the homepage and for a smaller one that appears on the content pages. The latter one could also be made to nicely integrate the title, breadcrumbs and horizontal line that all pages currently have.
Correcting the case in the logo, so that it says OpenACS instead of OPENACS would be cool (has been said here before I think).
I don't think that a prominently featured download button is a good thing. OpenACS is not something that can be downloaded and immediately installed easily - if you give newbies that impression by advertising a quick download it will only create frustration. Having links to demo or production sites on the other hand would be a great way to show off what can be done with the toolkit, and people whose appetite is wetted will more likely accept a longer installation procedure (and find the download on openacs.org, even if they have to click on "software" before or type in "download" into the search box).
And finally a question that I think has been asked here before but not answered yet: are the current design suggestions based on a fixed width? (I hope not)
