In response to Ralph, Alan and Mike:

I focussed especially on bibliographic searches and perhaps did not enough take into account other domains. So I can not really judge how ambigious index names can be and how serious this will effect the search results.

With respect to Dublin Core I would encourage to do things in line with Dublin Core but it was not my intention to use Dublin Core as THE one and only namespace.  Most of the 15 elements are are quite usable but these 15 elements are not enough and a lot of elements need a qualifier to be meaningfull. For example in case "DC.identifier.isbn" I would not use "identifier" as index name but rather "isbn".
Besides that there will be more and more DCMI application profiles introducing all kinds of other namespaces. Therefore - if namespaces are used to indicate the attributeset - I would strongly discourage the use of "dc" as prefix.

My proposal is the following:

1) Allow local and global namespace prefixes.
2) Let the server decide what to do with unprefixed index names. 
3) Let's try to define a list of unprefixed index names (instead of dc.xxxx) just to standardize the names (ambiguity is the clients risc).
4) Make sure Explain specifies the servers behaviour unambiguously 

Is this acceptable for everyone?