I have a very little baby -- she recently landed at the 2nd percentile at 7 mos. She started much higher and has consistently slipped.
Most of the time I don't worry at all as she's given me nothing else to worry about -- she's extremely alert, rolls this way and that way, sits up, laughs, plays, has a great attention span and is always in a good mood. However, she's not really a "good" eater (obviously). Also, both my husband and I are on the smaller side.
But every now and then like now I get concerned and would love to hear about people who have had similar experiences and how it all turned out...