The world's largest and most structured offest market is the Kyoto Protocol's Clean Development Mechanism. Offset critics claim that project documentation can be easily mispresented, that the program doesn't drive renewables, and that the financial benefits of carbon offsets prevent developing countries from advancing regulations designed to limit carbon emissions. CDM projects require independent verification - if these problems exist with independent oversight and verification, similar problems certainly exist in voluntary systems where there is no oversight.
Clearly it is appropriate to consider the impact of travel and what you can do to reduce that impact. It is better not to travel that to travel with purchased offsets. Maybe changing work practices, staff rotations, or sheduling meetings via web applications can reduce your organization's travel. If you do choose to purchase offsets for your travel, then you should really take the time to understand what you are buying. If you work to reduce the carbon footprint for your business operations and then take the time to document the reduction, you'll likely achieve real environmental benefits, you'll demonstrate an involved approach to managing the problem (anybody can buy offsets, reducing a carbon footprint takes involvement), you'll be able to communicate your accomplishments, and you'll probably improve the financial performance of your organization.