A.,
You may want to look at Stephanie's post, too. Her 2 1/2 year old is going through the same thing. All three of my children who have surpassed age 3 have gone through this. I asked my pediatrician about the first and she said it was totally normal at this stage, so I learned to expect it with the others. I have heard other moms talk about it, especially with children who were early talkers or very talkative.
Around age 2 1/2 - 3, they go through a phase where they think quicker than they can talk. It is a matter of learning to catch up. It isn't a disability and doesn't require intervention, just a phase. One mom said her son benefited when she would coach him to slow down and say one word at a time. It takes a few weeks to a few months to outgrow. If it has been over 6 months and it is still prevelant, you may want to consult your pediatrician.
Good luck,
S.