MechBgon, it turns out that each different type and model of M$ hardware product has a specific warranty duration, so you need to keep this in mind when you call. This table shows the various products and warranty durations:
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q173715
The rodents themselves, in addition to the PN, also have a manufacturing date code on the bottom label. It will be a 4 digit #, often in different looking print, like 0299 or 4101. The last two digits are the year and the first two are the week, so 41st week of 2001 for the latter date code. If they ask you the date you put the rodent into service, it needs to be within the warranty for that product, and not TOO far after the manufacture date.
As it turns out, the above referenced page says the "Intellimouse Explorer" has a "Lifetime limited warranty", which it may well need, as this model has a history of the cable crapping out from repeated bending where it enters the mouse body (inadequate strain relief).
Also, you can take some of the bite out of that $6 shipping charge by adding other items, like the
$1 heat sink/fans or the
various other mice that Directron sells for as little as $1 each.