If del.icio.us wants to limit notes to 255 characters, that's certainly acceptable.
Less acceptable is the fact that they neither enforce it in the user interface nor let you know about it.
They just amputate and leave you wondering.
(Bug report issued.)
Update: Henrik points me to the new-ish extension del.icio.us Bookmarks, which does both enforce and inform (yay!). On the other hand, it doesn't allow shift+tab to navigate backwards between fields in the post dialog. Bug. Report. Issued.
Update 2: Also via Henrik, I've seen notes run longer in a network feed, but the same ones are still cut in the user's feed and on the site. wtf?
Update 3: Apparently, this isn't new at all.