Wow, what a great idea!  Rent lenses by mail through RentGlass.com.  Right now they only offer Canon lenses, but Nikon lenses should be available anytime now - or 'Spring 2006', as the site says.  It looks like lenses start as low as $17 for a single week and $51 is the highest.  Obviously, higher end lenses cost more than the cheaper lenses.  And if you rent for 2 or 3 weeks, the per week cost is slightly discounted.

The renter pays the shipping each way - which is automatically insured - on top of the rental cost.  While lens bodies are said to show typical wear, the optics are guaranteed to be flawless.  Currently the site is all first come, first served, so there's no way to really schedule a specific time period to secure a lens.  I don't have anything to compare it against, so I can't say if the costs and setup are good or not, but it all sounds fair enough.  A great way to try before you buy, if you ask me.  I've got my eye on a couple I'd like to try out before I shell out big bucks for new glass.