If you are listed in your local MLS, most likely your property will automatically also show up in Realtor.com, yahoo real estate, msn real estate, and aol real estate. While this is true for most listings in most areas across the US, there are exceptions. Most MLS's feed all of their listings to the sites listed above via Realtor.com. If you fall into the 'majority' category, upon your agent entering your property to the local MLS, your property will also appear in the others mentioned above.
As a Broker, we also submit our listings to Trulia, GoogleBase, Point2, CraigsList, and about 20 others. This is a service we provide to promote and market the property as part of our seller representation. This is not something that automatically happens. It does take time and effort, and we have paid staff who are involved on a daily basis with internet marketing activities. I will also note here that we do less "print" advertising in favor of the investment we make to ensure agressive marketing via the internet. As a Broker, I can attest that our staff and labor expenses run high to pursue this. Even the sites we submit to that are without fees, cost us in labor costs. Not all sites are without fees, either. Our company position is that internet marketing is a better vehicle to reach serious buyers than print, and therefore we invest in that direction.
Your property will not 'automatically' appear on Trulia. Where your property appears (or not) on the internet is part of the marketing package used to promote your property.
Hope this helps.
As the other answers have stated, the listing in MLS does not mean your property will show up on many of the internet sites now used in sophisticated online marketing plans. One resource I use for my listings is Postlets. Their site makes it easy to enter property data once and it will be distributed to many sites including Trulia, Google Base, Oodle, Edgeio, Backpage, and others. These are all sites that potential buyers are perusing!