查询列表项并使用SharePoint Web服务与对象模型

My company is looking into writing a custom application that will need to perform many list item queries across multiple site collections. It will need to run for WSS 3.0 and it 'would be nice' if it worked on WSS 2.0 as well. It won't be designed for MOSS/SPS but again it 'would be nice' if it worked on these platforms. There is no restriction on which .NET version should be used for the solution.My company is looking into writing a custom app