Turn out there are a bunch of ways to do it, so far the easiest way I found was using a freeware product called Andrea Mosaic. Download from here. Yes there's a Mac version too.
You have decent control but rather than going through all the details I'll just let you reference the tutorial. I created the mosaic below after tweaking it in about 30-45 min. I have decent hardware for a desktop (8GB of ram, dual AMD core [blah, blah blah]) so I was able to recreate the mosaic fairly quickly. BTW - it helps to have a teenager who is tecky savy to build that machine - tigerdirect.com barebones kit is what we used. I think it was $450 two or three years ago, regardless it's a pretty happening machine.
The reason I bring that up is because your milage may vary on how long it takes to build a mosaic. For reference I added 473 photos on a mosaic of 2000 tiles. It has to do a bunch of optimization [more blah blah blah here] to create the mosaic. I sized the canvas at 1 meter. So I could put the below on the wall with no problems. My experience was the more photos the better - if you don't have a bunch let it duplicate the photos. I also set the blend rate/ modify original color at 100%.
In my case each mosaic was created in less than 3 minutes. So it took me about 8-10 tries to get what I thought was acceptable - that included a learning curve on how to use the software.
I'm going to try some things in PS, but the below was pretty easy. The only issue I had was I wanted a bit more control (as usual) on what was laid out where. That said, I'll do something and post it in a bit.
That said this is what I started with.
It created a file that was 26MB - looks like there's a limitation at photobucket as it wouldn't load a 26MB file. So this is a slightly smaller file with some loss of quality. Lean back a bit in your chair/step back and you'll see the original photo better.
When you zoom in you can see this (this was from the original 26MB file).
Getting in tighter...
