It worked fine in 1.0.3 (or whatever it was before 1.1.0), now it shows a 404 page from my localhost web server.
Yes, I did change project settings to point to the correct URL in the proper virtual host. It seems that the designer ignores these settings and uses localhost all the time. Here is the entry from the default virtual host in the server log:
::1 - - [17/Mar/2011:10:57:23 +0200] "GET /designer/preview.html HTTP/1.1" 404 634 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X) AppleWebKit/534.24 (KHTML, like Gecko) Sencha Designer Safari/534.24"
Project URL prefix is set to "http://sf_search/typo3conf/ext/sfp_jumpurlstats/resources/js/".