They will go to seed, Jocelyn. I am assuming that these are sweet or yellow onions from the store.
If you want seed from that onion, that would be the way to get it.
2010 onion
sets were grown in 2009, however, they were not given enough time to fully mature. Still, they are "keeper" varieties and have been stored through the winter for setting out about now.
Onion
plants, that we can find in the garden centers, were grown from seed sown sometime this winter in a warm area like southern Texas. They are pulled and shipped around the country.
Onion
seed was harvested in 2009, packaged, and made available this year. If you have a plant bloom and make seed, you can grow 2011 onions from that seed.
Steve
Welcome to TEG

!
edited to make that read that
this year's seed was harvested
last year. Sometimes, I'm kind of lost, too . . .