Siebrand formulated targets for 2009 for translatewiki.net . They are ambitious but doable and I think that he prefers to be overly ambitious then not to be ambitious enough.
Last week we were really happy that the target for the "most used" messages was achieved; 130 or more languages had at least 98% of the 472 messages localised. These messages are the most used messages and we consider that those languages without these messages localised have a serious issue with their usability for their language. The weird thing is that some languages that are actively localised do not support their most used messages 100%.
Today we are again two languages short of the 130 languages mark..
The reason for this drop in the numbers? Two messages got plural support, causing previous translations to be marked as outdated and no longer counting as translated messages.