There are several places where (thin) non-breaking spaces should be inserted, e.g., between numbers and units. Of course, there exist different spacing rules in different countries.
Up to now, inserting thin spaces still leads to problems: "nbsp" is generally too large, "thinsp" and "U+202f" won't be displayed in the wanted way using opera and so on; see e.g. [http://de.wikipedia.org/wiki/Wikipedia:Meinungsbilder/Typographie_(Zwischenr%C3%A4ume)#Browser-Unterst.C3.BCtzung] (german).
There are possibilities to display thin non-breaking spaces by using some html/css tricks, see [http://de.wikipedia.org/wiki/Schmales_Leerzeichen#.C3.9Cbergangsl.C3.B6sungen]. But that would complicate the source of articles too much.
At http://de.wikipedia.org/wiki/Wikipedia:Fragen_zur_Wikipedia/Archiv/2009/Woche_01#.26nbsp.3B (german, but with php source-code) the idea was given to use (localized) regexps for automatically inserting of whitespace in some cases. With this modification we could easily auto-insert even sophisticated things like [http://de.wikipedia.org/wiki/Schmales_Leerzeichen#.C3.9Cbergangsl.C3.B6sungen] without obfuscating the article source code.
Maybe such a thing would slow down the parsing of wikitext, so I guess it would be the best to implement the idea at test-wiki first. Somebody should profile the parsing after those changes. I could help in generating some fast regexps.