[This bug][1] is still prevalent, appears to be cache related. Two identical requests return different data: first request respects `jsonp` query parameter, all subsequent requests ignore it until (I presume) the server-side cache expires. This is the case for revisions 1.0 and 1.1.

These two requests in chronological order:

    GET http://api.serverfault.com/1.1/users/6246/top-answer-tags?key=P5K7zSMTYk-Has4Spe848g&jsonp=jsonp12980289083367&_=12980289093903232

    200 OK

    633 B
     
    538ms

    Cache-Control	private
    Content-Type	application/javascript; charset=utf-8
    Content-Encoding	gzip
    X-AspNetMvc-Version	3.0
    X-RateLimit-Max	10000
    X-RateLimit-Current	9999
    X-AspNet-Version	4.0.30319
    Date	Fri, 18 Feb 2011 13:27:09 GMT
    Content-Length	633
    X-Antivirus	avast! 4
    X-Antivirus-Status	Clean
    Request Headersview source
    Host	api.serverfault.com
    User-Agent	Mozilla/5.0 (Windows; U; ...
    Accept	text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8,application/json
    Accept-Language	en-gb,en;q=0.5
    Accept-Encoding	gzip,deflate
    Accept-Charset	ISO-8859-1,utf-8;q=0.7,*;q=0.7
    Keep-Alive	115
    Connection	keep-alive
    Cookie	__utma=81883924.1801528020.1274785032.1286198914.1286791946.7; __utmz=81883924.1286791946.7.5.utmcsr=google|utmccn=(organic)|utmcmd=organic|utmctr=nginx%20does%20not%20pass%20request%20to%20apache; __qca=P0-1856570125-1274785031953
    x-insight	activate
    Pragma	no-cache
    Cache-Control	no-cache
    
    jsonp12980289083367({
      "top_tags": [
        {
          "tag_name": "linux",
          "question_score": 2,
          "question_count": 1,
          "answer_score": 37,
          "answer_count": 19
        },
        {
          "tag_name": "apache",
          "question_score": 4,
          "question_count": 1,
          "answer_score": 35,
          "answer_count": 21
        },
    ...


(sorry for the verbose headers!)

    GET http://api.serverfault.com/1.1/users/6246/top-answer-tags?key=P5K7zSMTYk-Has4Spe848g&jsonp=jsonp12980289083367&_=12980289093903232
    	
    200 OK
    	  	
    611 B
    	
    233ms
    	
    ParamsHeadersPostPutResponseCacheHTMLCookies
    Response Headersview source
    Cache-Control	private
    Content-Type	application/json; charset=utf-8
    Content-Encoding	gzip
    X-AspNetMvc-Version	3.0
    X-RateLimit-Max	10000
    X-RateLimit-Current	9998
    X-AspNet-Version	4.0.30319
    Date	Fri, 18 Feb 2011 13:27:37 GMT
    Content-Length	611
    X-Antivirus	avast! 4
    X-Antivirus-Status	Clean
    Request Headersview source
    Host	api.serverfault.com
    User-Agent	Mozilla/5.0 (Windows; U; ...
    Accept	text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8,application/json
    Accept-Language	en-gb,en;q=0.5
    Accept-Encoding	gzip,deflate
    Accept-Charset	ISO-8859-1,utf-8;q=0.7,*;q=0.7
    Keep-Alive	115
    Connection	keep-alive
    Cookie	__utma=81883924.1801528020.1274785032.1286198914.1286791946.7; __utmz=81883924.1286791946.7.5.utmcsr=google|utmccn=(organic)|utmcmd=organic|utmctr=nginx%20does%20not%20pass%20request%20to%20apache; __qca=P0-1856570125-1274785031953
    x-insight	activate
    Pragma	no-cache
    Cache-Control	no-cache
    
    {
      top_tags: {
        0: {
          tag_name: "linux",
          question_score: 2,
          question_count: 1,
          answer_score: 37,
          answer_count: 19
        },
        1: {
          tag_name: "apache",
          question_score: 4,
          question_count: 1,
          answer_score: 35,
          answer_count: 21
        },
    ...


  [1]: http://stackapps.com/questions/1449/api-selectively-ignores-jsonp-parameter-havok-ensues