I have been spending a lot of time on timezones and I'll share what we've been doing ...

First of all, everything I am doing is with the IANA timezones so this would only apply if that's what you are doing as well. There is a shapefile that geographically defines those timezones as polygons at efele.net/maps/tz/world/[/url]. I downloaded the archive from there and imported the data into an on-site SQL Server using a little free client tool called Shape2SQL. (I still need to figure out how to do this through an automated means.)

From there, I can take geographical coordinates to determine the IANA timezone through a query against that imported data. We also store all the IANA definitions in SQL as well but the conversion logic lives in a .Net assembly ... that kind of logic just doesn't fit as well in t-sql.

There may be a vendor who would do this all and sell it as a service that would work for you, but I have not found it for our situation. I had a meeting with some licensing people at Microsoft (Bing Maps) last week because we need to geocode a few million addresses a year and I know for a fact they don't do anything with timezone resolution because they were worried that was what I was asking them for. Google has some API's that will return timezones but my understanding of their licensing model is not compatible with what we need to do so I have not pursued that path.

This is by no means meant to be exhaustive ... it's just what I know about converting geolocation to timezone in the midst of a project I am working on. Good luck!