I don't know about now, but there was a time when express flights were not slotted. That's what caused the horrible delays a few years ago..pre 9/11. I lived and was based there at the time. I just don't think the money was there and well the rest is history.
That's no longer the case, thanks to the massive congestion caused when the removed the slots for Express flights and US/AA/DL/CO all overran the airport with RJs to everywhere.
There are, though, seperate pools of slots for mainline flights and commuter flights. Commuter flights can of course use mainline slots, but not vice versa. At LGA, I believe US owns 226 air carrier (mainline) slots, good for 113 roundtrips. As US only flies about 51 daily mainline roundtrips at LGA, they're using over half of their mainline slots to fly Express flights. You can clearly see that they've got more than plenty of excess mainline slots should they decide to revert back to mainline on some route, the slots would not be an issue. AA and DL, on the other hand, use the vast majority of their mainline slots to fly big planes, which is why US/AA/DL are all almost exactly the same size at LGA, when US has far more slots.
And, of course, just to be completely clear, US no longer *owns* any commuter slots at either LGA or DCA, having sold them to and leased them back from Republic as part of the -170 sale/disaster. They are guaranteed the right to lease those slots (at I'm sure a healthy profit for Republic), though, so they won't be used by another airline, and US has the right to buy the slots back (at I'm sure a healthy profit for Republic) in two years.
And the other thing--the commuter slots at DCA may be used for 72-seat equipment and below, like the EMB-170, which US makes good use of, as they use their mainline slots at DCA for actual mainline flights. At LGA, on the other hand, the commuter slots can only be used for 50-seat equipment and below, so any 70/72-seat planes at LGA would need to use mainline slots. Again, not a problem for US, but that's why you don't see AA or DL flying CRJ-700s into LGA.