Typically single vendor. Where we have a special application, or can get a higher quality vendor in for the same price, we'll go dual vendor -- but ONLY dual. Desktops we keep all the same vendor, because they're more loyal to us and provide better support when we ring them up and say that we have several hundred of their systems and they need to fix this now.
We also tend to buy the same model for as long as we can -- parts are more interoperable and it's easy to extend the life of some systems past the warranty if we have 100 to pull parts from.
Server-wise, we tend to stick with Dell and Sun. Dell is the best per-core price usually, and they have a good rap for getting parts here within 4 hours, plus tech if necessary. However, their build quality and reliability sometimes leaves much to be desired. Frankly, we prefer Sun hardware if we can afford it at any one point because the build quality is higher and their storage products have amazing density for the price as well as features that competitors charge several times as much for. We've resisted adding IBM, HP, and individual storage-only vendors to our picture even when they've had compelling products because we'd rather stick with products from the two vendors we already have. Dell and Sun both provide great tools to manage your inventory and support contracts. HP does not as far as I know, at least through the reseller that we buy through. (HP themselves is not an approved vendor for my workplace. We have to buy products through a reseller.)