Hi fireberd350,
I also was surprised at he number of latex dog beds a google search turned up.
I have no idea what thickness or firmness may be best for any particular dog but the latex itself would be a good quality material and you can differentiate them in the same way as a latex mattress for humans. I would probably use a good waterproof cover of some type. Post #6 here has more information about the different types of latex but all of them are durable materials compared to other types of foam.
I doubt you would need more than around 3 or 4" of latex unless you really wanted to spoil the dog 
If any of the forum members have any experience with latex dog beds it would be great to hear some feedback.
Phoenix