Basically, Serendipity should support the Flock browser.
Sadly, due to a bug in their parsing/sending routine, applications that rely on PEARs XML-RPC extension won't work with flock. Serendipity falls under that category. It would work without flock auto-detecting a blog, but because flock's autodetection is blocking to successfully add Serendipity, this is a devil's circle.
This flock bug had been reported in February, but sadly no progress has yet been made. All the people who'd might like to use their Serendipity Blog and the XML-RPC posting plugin with Flock, please raise your kind voice here:
I am pretty sure the nice people of Flock will report to popular demand - I'd really like to see progress in this issue :)