For as impossible as it seems, I wouldn't be surprised if at some point most all 5-letters in .com are taken too ... there are nearly 12 million possible combinations (much more if one includes hyphans and numbers), but at current domain name registration rates it's quite foreseeable that most all 5-letters will be registered eventually...
On a related note, each letter in length increases the possible number of combinations by a multiple of 26 ... so, in my view, 5-letters will mark the end of the "collect them all" mentality ... 6 letters (over 300 million combinations) and beyond (billions upon billions of combinations) will continue to remain relatively sparse well into the distant future.
Ron