Seems I have narrowed the problem down to Global.UrlRewriteProvidet.ConvertToExternal (from EPiServer.Web.UrlRewriteProvider).

Some background: I'm importing pages from a 4.61 site where MasterLanguage=EN and pages have LanguageBranch=EN.

The 6 R2 site I'm importing into has MasterLanguage=NO, and EN is disabled. (I'm using a SQL script hack to change the page language after importing: http://labs.dropit.se/blogs/post/2010/11/01/Move-content-to-another-language-in-EPiServer.aspx)

For each imported page, I do the following:

var ub = new UrlBuilder(pageData.LinkURL);  
// typically "/Templates/Pages/Article.aspx?id=2048"

ub.QueryCollection["epslanguage"] = pageData.LanguageBranch;
// pageData.LanguageBranch="EN"
// ub="/Templates/Pages/Article.aspx?id=2048&epslanguage=EN"

Global.UrlRewriteProvider.ConvertToExternal(ub, pageData.PageLink, Encoding.UTF8);
// After ConvertToExternal, most pages returns ub="/somepath/the-real-urlsegment-of-the-page/".
// But a few problem pages returns ub="/somepath/placeholder-for-page-of-type-/"

I have not been able to determine why this happens with only a few pages. 

Edit: Googling for "placeholder for page of type", I see that several live sites (cms 5 and 6) have had their urlsegments changed like this.