There are a few points that disqualify the mini from serious graphic work.
1. graphics processor integrated into motherboard and not powerful. CS4 now uses the graphics processor more than CS3 does.
2. It 2Gb is the maximum RAM on a mini, then this is not enough for Photoshop. For Photoshop you want 3Gb of RAM.
You CAN use it. It's better to use a mini and a good display than a 24" iMac with those glassy, glossy displays.
But don't think Photoshop will run very fast.
What's your average file size?