Neat idea. Could maybe leverage other sources (eg Delicious 'notes' fields, esp. if someone self-notates as a 'summary') until someone first writes a tight proper-style summary. Choice of hard limit for summary size is somewhat arbitrary but will have important influence on culture that emerges.

Getting ahead of things, but an idea for v2: would be interesting to have summaries that apply to entire 'site' (by domain or URL-prefix) or 'site-subsection' (by URL prefix or pattern). When viewing an individual URL summary, you might also want to see (below it) the summaries of each containing context.