The official cap for training is indeed 300, however I think that you can go past this with Racial abilities (i.e. Gnomes get +5 Engineering at all times) as well as special effects that items may have (i.e. Herbalist Gloves add +5 ability to Herbalism when equip). There must be some items that allow for bonus points in tailoring, or any other profession that requires more then 300 skill points in a profession.