Dual GPU nVidia cards are becoming more common, although even on a global scale they are still a very minor part of the market. This leaves plenty of time for ATI to catch up, and lately GeCube has introduced one of the first dual GPU Radeons. Based off the X1650 XT, the new dual chip board is dubbed the "X1650XT Gemini Twin 2". Quite a name for a card, but for this much hardware it's expected:

The new GeCube Gemini 2 Radeon X1650 XT graphics card carries two Radeon X1650 XT (RV560) graphics chips each of which has 24 pixel shader processors, 8 vertex shader processors, 8 texture units, 128-bit memory interface and supports Avivo technology. The new graphics board uses CrossFire technology to take advantage of two GPUs in order to increase performance and/or image quality in games, however, in future end-users will be able to utilize the power of the second GPU for physics calculations in future titles.
Each GPU has an independent 256MB of RAM, and it's unclear how much having a smaller amount for each GPU would impact performance. Neither pricing or compatibility is made available yet, though you can expect it to be pretty expensive.