I've done the Playa to Belize trip by bus several times, most recently in June. Here are a few tips not covered yet in other postings:
* You take an ADO (Autobuses de Oriente) bus from Playa to Chetumal, but note that there are now TWO ADO terminals in Playa. The older one, near the ferry landing, is NOT the one you want. The newer one -- it opened earlier this year -- is a taxi ride away.
* Buses from Playa to Chet leave about every two hours during the day. It takes about 4 1/2 hours with one stop. I think the last trip I paid about US$17, but it varies depending on the exchange rate and where you buy it. Buses are fairly modern, with A/C, reserve seats and videos.
* In Chetumal, you will arrive at the main terminal. Most buses to Belize -- Novelo's dominate this route -- leave from the New Market terminal BUT they stop briefly at the main terminal, so you can catch them there. It's about US$2 to Corozal Town. The bus is a big help in getting you across the border, as it stops first at the Mexican check point, where you may have to pay your Mexican tourist fee if you haven't already (about US$20) but don't have to take your luggage. You then reboard bus and cross the bridge to Belizean customs and immigration, where you do have to take your luggage. Then reboard the bus to go into Corozal (the whole process from Chetumal to Corozal takes less than an hour usually). The driver's assistant will assist you with luggage and may help with Mexican officials.
Also keep in mind the difference between Mexican and Belize time (!) For one thing, Belize doesn't follow Daylight Savings Time.
In Corozal, you can fly (Tropic or Maya Island) direct to San Pedro. The airstrip is just south of town. One-way to San Pedro is US$35-$40. Or as Gaz says you can take the ferry. Lady Lowe ferry no longer runs to San Pedro, but Thunderbolt does.
By the way, Corozal Town is a nice little place. Not a whole lot to do, but it's safe, friendly and an enjoyable place to just look around. The setting on Corozal Bay is beautiful. And hotels and restaurant prices are a fraction of what they are in San Pedro. At best restaurant in town, Cafe Kela, you can get a great meal for US$5, or for really cheap Mexican-style food try Cactus.
A few car rental places in Cancun/Playa will let you take the vehicle into Belize, but if you are going to Ambergris Caye I don't really think it's worth the expense and hassle to rent a car, pay for Belize insurance and then leave it on the mainland.
Belize books and publications by Lan Sluder:
Belize First Magazine (http://www.belizefirst.com
Fodor's Belize & Guatemala Guide (new 4th edition coming this summer)
Adapter Kit: Belize (the first and only comprehensive guide to living, retiring, working and investing in Belize -- a best-seller on Amazon.com)
San Pedro Cool (the first and only visitor guide to Ambergris Caye)
Belize First Guide to Mainland Belize
UpClose Central America
Belize Book of Lists