I have seen an issue with twitter.com and facebook. I cannot stay logged in past the time when I close the browser window. This started happening about 2 weeks ago with the nightlies and has not been corrected. I can log in but if I close the window and reopen the page, I am now logged off. I have the cookies for both pages set to keep me logged in. THis may be happening on other sites, but these are the 2 I have noticed it on. Version 5525.27.1
Could you please isolate between which exact revisions this issue started to happen for you? It is quite surprising that any changes in WebKit could affect this, so I suspect that some other change in your setup may be the culprit.