I think somebody asked something like this in the fruit and orchard forum. The answer was that roots of the trees would be drawn to the top layers of the bed and would interfere with plants with shallower roots. Plus, the tree would shade most of the bed for a lot of the day.

All in all, it might work, but you would have to be careful with the plants you choose.

I'm doing this, but with espaliered trees.
I have strawberries growing underneath the trees - just to keep weeds down and not to waste any space.

Here is a link that might be useful: My Blog

What would cause the roots of the trees to be drawn to the top layers of the raised bed?

Anyone know why the roots would be drawn up?

I was thinking of making the raised beds probably no higher than a cinder block.

I'm no expert, but I imagine having water and fertiliser/compost near the surface - which of course you need for seedlings and short-rooted plants - would draw the tree's roots upwards.

If that's the case, wouldn't normal composting and mulching still bring the roots up if planted in the ground (as opposed to a raised bed)?

Also, does anyone know whether regular additions of compost and mulch will affect the soil level/height? If it is going to keep raising the soil level, necessitating the need to make the raised beds taller to retain the soil, I'd probably be better off planting them in the ground.

I can't help you with most of your questions but I can tell you that regular addition of compost will not tend to raise the bed height mush just maintain it. you would be suprised how much the soil level sinks in one year.