I recently wanted to get started doing a bit of iphone development and because our good friends at apple have made it a requirement to develop on a mac... I found myself wanting to find the cheapest one I could find.
After doing a bit of searching, I found that a Dell mini 9 will take a default install of Snow Leopard with a bit of help from a tool called NetBoot Installer. Using NBI, a purchased copy of OS X and an 8GB thumb drive you can setup and install a very nice little "Hackintosh".
While I could rehash the instructions, I think there is a fine one here.
I was a bit skeptical when when doing this because I had no experience with an Atom processor and didn't know what to expect. So, after install I put it through it's paces, including installing and running Photoshop, running a Skype video conference while also installing other applications. For the most part this is a very capable system, I'd even go so far as saying that the experience is as good as my wife's Mac Mini. The real constraint is when you are jumping between several big apps... like Photoshop and several open Chrome windows. None the less, this is certainly a very good and cheap alternative to buying a Mac Mini.
I purchased the Dell mini 9 on Ebay for $175 and bought the 5 seat OS X license for $50. So all in all I'm in $225 for a very capable and very mobile Mac.
UPDATE: so about three weeks after I purchased the Dell, the graphics card went out... So I'm not having to purchase a three year Dell warranty for $120. (I could have just bought the 1 year, but the difference was only $50 between the 1 and 3 year price. But the nice thing is that I'll have a mac guaranteed for 3 years. Hind site being 20/20 I think I would have just bought a new Dell mini 1012 which would have come with a 3 year warranty for only a bit more $$. :( Stupid hind site...