Don't hold your breath. Although GM is shortening the length of time
between new body styles, we are only 7 model years into the latest
cycle:

1973 -1987 = 15 years

1988 -1998 = 12 years

1999 -2005 = 7 years

In my opinion, your best buy is a 2004 Silverado, because GM has
increased prices and lowered quality and performance in its 2005
Silverados:

There are 3 differences in standard equipment for 2005:

2004                                    2005
4-wheel disk brakes           Front disc/rear DRUM brakes

P235/75R16 tires               P245/70R17 tires
Max rating 2271 lbs.           Max rating 2205 lbs.

Rear axle 3.42 ratio            Rear axle 3.23 ratio

In the 2005 Silverado, the brakes have been compromised, the
lower-profile tires have lower carrying capacity and a rougher ride and
the standard rear axle provides less power, but it does deliver 1 mile
per gallon more highway mileage.

If GM continues its downgrade spiral, the next Silverado body style will
have a more expensive price, more plastic, less stability and its
components will be further cheapened and less reliable. The rebates,
financing deals and prices of the better-quality 2004 Silverados make
them a better choice.

You can see where Silverado styling is headed by looking at the styling
of Chevrolet's highly-successful Transformer-looking, multi-function
Avalanche. In a few years, the plastic cladding and Buck Rogers styling
of this Tonka truck will spill over into the Silverados. If that's the
kind of truck you want you can wait a few years. If you want the last of
the good Silverados, you can buy a 2004 model.

We are only 7 model years into the latest Silverado cycle. Now it
appears that there may be 2 more years to go on the current design.

Here's where Silverado is probably heading:

2008 Chevrolet Silverado: Redesign.

http://motortrend.com/future/spied/112_fvfmain/