create a backup immediately!
Exactly. RAID is not a backup and it is always possible that for any one of a catalogue of reasons (e.g. virus, RAID controller corruption, both drives failing, theft, fire, stupidity - need I go on?) you can lose the data. ALWAYS take regular backups and store the drive in a safe place, preferably offsite.
And most importantly for now, take a backup before attempting to replace the failed drive, even if it's just your most important data to a desktop or laptop.
Edit: Oops, I've just noticed that the original drive failure was in May so my warnings were a I'm a bit late for that one. My backup comments are no less relevant now though...