I have been asked many times about a possibility to get some good training for ApEx. I think, there is an oracle two-day training on basics. However, what people want is usually more than that. Some weeks ago, the European APEX Training Days of the APEX Evangelists was announced. This will be a 3-day training covering advanced ApEx issues like ajax, security, customizing templates and many, many others.
If you are interested, you can benefit from the knowledge of John and Dimitri.
John is Oracle APEX developer of the year 2006 and is the author of the book "Pro Oracle Application Express" - a person with an Avatar level in ApEx.
Dimitri is a well known contributor on the OTN Oracle APEX forum and speaker at Collab'07. He is a creator of the popular dgturnament application and has many years of experience with Oracle.