Your soil will be slow to warm up and toms don't like cold soil. I'd pull the leaves back from the spots I was going to plant and maybe put down some clear plastic until things warm up. Then you could remove the plastic, plant your seedlings and pull leaves next to, but not touching, the plants. The leaves will break down over time and feed your garden. In the mean time you won't have weeds. Don't leave the leaves piled deep into next fall and winter or you might have vermin.
Jim

Those leaves will eventually rot into the soil and make it soft, porous and more fertile. They are earthworm's favorite food, so that population will explode, which is good news too.

The only hesitation I would have is that if they are large leaves, like, say, maples, the wind will blow them around and they could easily cover up your baby tomato plants and shade them out or even smother them.

If you had done this last fall, you'd be okay because the leaves would have begun to rot in by now, but since it's now, I'd recommend that you till or turn them into the soil first.

Short of that, you could rake them back onto your lawn, pick them up with your lawnmower bag and then use them for mulch around your plants. The lawnmower blades will chop them into smaller pieces and in that form, they are ideal mulch.

Either way you go, you can't lose.

Jimm makes good points. In my area, we are more worried about keeping the soil cool than getting it to warm up. :)

There were a couple of long discussions recently over on the Soil & Mulch forum about the effects of using lots of uncomposted leaves in the garden beds. You might want to browse through them because several problems were discussed in detail - soil pH effects, fungus problems from the uncomposted leaves, and nutrient depletion. Composted leaves are great stuff for the garden but uncomposted could be problematic.

Aside from the winds, as they decompose they can temporarily tie up some nitrogen but that can be compensated for by adding lots of extra nitrogen to them. Speeds decomp and helps the plants - but with tomatoes you have to take care to avid using excess nitrogen too. So its a balancing act.

I agree with the cold soil problem too. Mulches are usually only applied AFTER the soil has come up to warm temps. May not be too much of a problem in your zone but do so with care.

If it were me I'd likely rake them aside and let them compost in a pile for a bit, let the soil warm, plant, and then slowly bring them back in to act as mulch.

JMO

Dave