I'm lucky as my 5-year old just hits the 48" mark. We went 2x this summer, the first time, some times he made it and some times he did not. It depended on the ride and where he stood - he was that close. The second time, we went to the wristband place where you enter the park, and he was not tall enough. So I tore it off, and he made Flight Deck, Adventure Express, The Blue Racer, but not the Red Racer. So we went to Nickelodeon, where he was measeured and made the 48" cutoff - so we were set the rest of the day. He rode all the rollercoasters he could except for Vortex (his choice) and Son of Beast (my choice). He enjoyed The Beast, but did not want to go again as I think he got banged around a little.
He's up for it next year though.
My daughter was 52 1/2 inches, so perhaps next year she will be able to ride all the coasters, and I will be able to ride Firehawk, FOF Invertigo, and even Diamondback for the first time. She rode Son of Beast in 2007 and she wanted none of it this year. She did get up the courage to ride Vortex though, and eventually rode it 3x.