I do development for Windows, Windows CE/Windows Mobile, Symbian, Mac OS X, AIX, Solaris and Linux. The software is an ERP/CRM package aimed at mid-sized and small companies (single-user up to 1000 concurrent users). The C++ core engine is divided into three major parts; a virtual machine for the business logic, a very fast and scalable database engine and a cross-platform user interface engine. The solution can also use SQL Server and Oracle database engines.
For this I use a MacBoox Pro 2.33GHz dual-core with 2Gb of RAM. We have a single code base, and can start remote builds on the other platforms very quickly (largely thanks to a subversion-based build system developed in-house). On my development machine I typically have Xcode, Visual Studio 2005 and SQL Server 2005 running (through Parallels Desktop). When I'm working in Visual Studio, the changes are stored in the same folder as for Xcode, making it very easy to test fixes to cross-platform bugs.
I've found that the equipment is less important than the software tools you have to support the needs of the software you're developing. I've been using Intel Macs since they were launched, and despite my current rig being significantly faster than the one I used more than a year ago, I can't remember being frustrated with performance issues. Quite often I've found myself working with AIX or Symbian issues from my Mac, while being connected over a slow GPRS line, without speed being an issue.
That said, I'm not the type of programmer that does nothing while my code is compiling. I've learned to work on other things on my task-list instead of just waiting on the computer finishing some work - multitasking is not only for silicon chips ;-)
With good tools for remote builds, remote debugging and testing, the development rig doesn't have to be a stationary monster machine. Using a laptop for development opens up a whole new world of possibilities to sit down and work on that great idea, wherever I am and whatever network connection I have available (I always carry my cell-phone with me to give me a quick way to get online).
Compared to any previous development I've been involved with, my current setup is several orders of magnitude more efficient; I can get more work done [in a given timeframe] in this setup than I've ever been able to before.