Request for User talk:Rillke (see below)
Result:
http://commons.wikimedia.org/w/index.php?title=User_talk:Rillke/Discuss/2011&diff=prev&oldid=55209147

Script:
...
		edit['appendtext'] = '{{subst:test}} Bug 29278 -- ~~~~';
		params.redirect = '';
		params.format = 'json';
		$.ajax({
			url: this.apiURL,
			cache: false,
			dataType: 'json',
			data: params,
			type: 'POST',

...


Response-Header
Server	squid/2.7.STABLE9
Date	Mon, 06 Jun 2011 18:10:16 GMT
Content-Type	text/html
Content-Length	3023
X-Squid-Error	ERR_ZERO_SIZE_OBJECT 0
X-Cache	MISS from sq34.wikimedia.org, MISS from knsq30.knams.wikimedia.org, MISS from knsq29.knams.wikimedia.org
X-Cache-Lookup	MISS from sq34.wikimedia.org:3128, MISS from knsq30.knams.wikimedia.org:3128, MISS from knsq29.knams.wikimedia.org:80
Connection	close
Request-Header
Host	commons.wikimedia.org
User-Agent	Mozilla/5.0 (Windows NT 5.1; rv:2.0.1) Gecko/20100101 Firefox/4.0.1
Accept	application/json, text/javascript, */*
Accept-Language	de
Accept-Encoding	gzip, deflate
Accept-Charset	ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive	115
Connection	keep-alive
Content-Type	application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With	XMLHttpRequest
Referer	http://commons.wikimedia.org/wiki/User:Rillke
Content-Length	284
Cookie	centralauth_User=Rillke; centralauth_Session=<removed>; commonswiki_session=<removed>; hidesnmessage=1; popTz=0; vector-nav-p-tb=true



Parameterapplication/x-www-form-urlencoded
action	edit
appendtext	{{subst:Test}} Bug 29278 -- ~~~~
format	json
redirect	
summary	We have a sandbox for nonsense-edits; or use the preview function if you want to test.
title	User talk:Rillke
token	<removed>+\
watchlist	preferences
Quelle
action=edit&summary=We+have+a+sandbox+for+nonsense-edits%3B+or+use+the+preview+function+if+you+want+to+test.&watchlist=preferences&title=User+talk%3ARillke&token=<removed>%2B%5C&redirect=&appendtext=%0A%7B%7Bsubst%3ATest%7D%7D%0ABug+29278+--+~~~~%0A&format=json



From the HTML-Response: 
Request: POST http://commons.wikimedia.org/w/api.php, from 91.198.174.40 via sq34.wikimedia.org (squid/2.7.STABLE9) to 10.2.1.22 (10.2.1.22)
Error: ERR_ZERO_SIZE_OBJECT, errno [No Error] at Mon, 06 Jun 2011 18:10:16 GMT