Well from my experience so far I think the following things are absolutely vital:

• Powerful development machine
• Dual monitors (at least 22" or 24")
• Decent chair
• Unfiltered internet (I don't know how many useful sites I've seen blocked by WebSense)
• The ability to decorate/furnish your workspace (it's amazing how much this can help you be comfortable/at east)
• Good support servers for version control, wiki, backup, testing with other applications if your solution stack includes several, etc.