I got bit by a small gotchya while setting up a fresh Sitecore 7 install to use SOLR today. In SOLR, I left the core named the default "collection1" instead of renaming it to "itembuckets" as is recommended in the documentation. Well, if you want your core named something other than "itembuckets", you'll need to edit the index definitions in Sitecore.ContentSearch.Solr.Indexes.config to change the core names to reflect your core names.
The error I received was:
Connection error to search provider [Solr] : Unable to connect to [http://localhost:8983/solr]
I kind of wish the error could tell me it couldn't connect to the core I had specified in the .config file, since SOLR itself was running fine and going to the URL shown in the error message worked fine. But if you see this error in the future, you might check that your SOLR cores are correct in your .config files, anyway.
Thanks to Stephen Pope at Sitecore for making several suggestions on Twitter - one of which led me to discover this!
Next on my TODO list - get the Autohaus demo running with SOLR. Will post again once I know if it's possible and how to do it!