To quote Wikipedia:
As of 14 June 2007, all new mobile phones applying for a license in China are required to be able to use a USB port as a power port for battery charging. This was the first standard to use the convention of shorting D+ and D-.
In September 2007, the Open Mobile Terminal Platform group (a forum of mobile network operators and manufacturers such as Nokia, Samsung, Motorola, Sony Ericsson and LG) announced that its members had agreed on micro-USB as the future common connector for mobile devices.
On 17 February 2009, the GSM Association (GSMA) announced that they had agreed on a standard charger for mobile phones. The standard connector to be adopted by manufacturers including Nokia, Motorola and Samsung is to be the micro-USB connector
As such, you could expect any mobile phone USB charger to work for any mobile phone designed since these agreements were made - I admit to having far too many USB phone charges lying around, and have seen no issues charging Nokia devices with HTC chargers and vice-versa.
Generic, third party, "compatible" chargers might be another story, and it would be advisable to check the output current doesn't exceed 500mA.