SQL Server 2008 R2 on Windows 7 x64 Professional - what are the downsides?
Your going to run into occasional memory and thread handling issues.  I run 2008 r2 Developer (which is basically enterprise) on Win7 Prof (64x) on a development machine* for writing code.  I do not know the exact reason but SQL acts funky sometimes and you need to either restart service or box.  Ex. No queries will be running and cpu will be pinned at 50% with SQL proc that if you check with ProcExplorer isn't doing anything. If this is something you can live with doing every so often then maybe you can get away with it.  On the other hand if you need high up-time then spend the money on server edition. 

*I run like this because Visual Studio runs like crap on server and I like a portable compact dev setup when I'm on site.