Depends on how long you go in between upgrades.
In some sense, it is good to start with the "newest" since it will take the longest to be obsolete. But you have to worry about stability and the possibility of being axed (like the way HD DVDs are done for against BlueRays now)
I would certainly not call VS2005 obsolete. VS2003 yes (even though we still get plenty of question about it from people stuck with .net1.1)
I would also factor in other items when making this decision - admittedly, some consider 2008 "too" bleeding edge - whatever that means, however - with certain technologies, such as LINQ to SQL and LINQ to XML, stepping straight into 2008 might provide huge leaps in ease of programmability that may mean that applications can be developed faster using 2008 than they could with 2005.
Of course, if your apps don't lean on the ability to pull data out of databases or the ability to easily pass data back and forth between themselves or web services, then some of the newer technologies of the 2008 incarnation may not be of any significant benefit to you, in which case 2005 may be the better way to go.
I certainly wouldn't say "don't pick 2008" but I equally wouldn't say "2005 is adequate". It depends entirely on your programming and application needs and your budget. You can't really make a satisfactory decision without knowing all the factors that comprise the choices.
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