You can spend your time searching eBay for an old Intel Mac but in my experience it a false economy (especially if it's more than 3 year old).
One tip for anyone looking for a
secondhand Mac, as I did in the late
90s during the last economic downturn,
buy computers from graphic design
agencies or software houses that have
gone bust.
Good
Mac Mini - It'll do every thing you need (albeit not very fast). You might consider extra memory and if you have a a spare keyboard and monitor (or KVM switch) your away.
Better
New MacBook - released yesterday (2009-06-08) the new MacBooks are reasonably priced and a great option especially if you want (or are replacing) a laptop. Again RAM is a consideration and if you plan on long coding sessions a decent keyboard and monitor are essential.
Best
Dive right in and get a iMac (20" or 24")! I suggest you only do this if you want to replace an older non-Apple box. The calculation you have to make is: How much would I spend on a new PC plus how much would I spend on my development Mac. If the total is even close the price of an iMac it's an option.
You'll have the option of virtualized Windows, (unlimited) Linux installs, Boot Camp if your a Gamer and Mac OS X for your iPhone/Cocoa development.
If money is no problem you can even go for a Mac Pro.
My system is a Mac Pro - I run Windows XP, Vista and 2 x Cent OS's (simultaneously). Also I have Windows 7 Beta and a few Ubuntu's for ad hoc work.
It works great - but wont be for everyone taste. I replaced my Uni set up of: 1 x Mac, 1 x PowerBook, 1 x Wintel box and only using Linux on campus with MacPro and iPhone (I still have the PowerBook, but it's hardly ever used).