Some of our setups use Solr in a SPNego/kerberos setup (we've done this by adding our own filters to the web.xml). We have an issue in that SPNego requires a negotiation step, but some HttpSolrServer requests are not repeatable, notably the PUT/POST requests. So, what happens is, HttpSolrServer sends the requests, the server responds with a negotiation request, and the request fails because the request is not repeatable. We've modified our code to send a repeatable request beforehand in these cases.
It would be nicer if HttpSolrServer provided a pre/post callback when it was making an httpclient request. This would allow administrators to make changes to the request for authentication purposes, and would allow users to make per-request changes to the httpclient calls (i.e. modify httpclient requestconfig to modify the timeout on a per-request basis).