Doesn't Rackspace provide tech support? I thought that was part of their service that they are so proud of.
Try re-installing WordPress from a fresh download. On the server keep wp-content, your htaccess file and your wp-config file intact. Replace everything else. Delete everything first, then reupload. The techinique I use to do this is to download a fresh version of WP, extract it on my local computer, remove the wp-content folder, and the wp-config-sample.php, rezip it and then upload the compressed file to my server and extract it there. Uploading the zipped file is faster than uploading an umcompressed file directory.
Then, ensure that your plugins folder is deactivated by changing the name to something like /plugins-hold/. Try loading the site. In doing so, you will be loading the site with only WordPress Core and whatever theme you have activated (by changing the name of the plugins folder, WP won't load the plugins because it can't find them).
If it loads, then change the plugin folder name back and start reactivating your plugins one at a time. This might fix your issue, it might not, but at least you are moving another really solid step through the trouble shooting process.
