Why is my captivate able to run flash on android 2.1?
Samsung put the "Flash Lite" plugin on the Galaxy S for 2.1.  It supports the basics.  This blog shows some devices that use the plugin: http://www.adobe-flashlite.com/

Edit: Adobe now has a list of devices that can run full Flash.  http://www.adobe.com/flashplatform/certified_devices/