Upgrading to Microsoft SharePoint Server 2010 from SharePoint Server 2007 provides significant enhancements in enterprise search capability. This includes many improvements for end users when they issue search queries and view search results. You can find a summary of these enhancements in What's new in enterprise search (SharePoint Server 2010).
However, due to policy or resource constraints, organizations sometimes aren't in a position to complete an upgrade all at once. If this sounds like your organization, you can still set your end users up to take advantage of search capabilities of Microsoft SharePoint Server 2010 from your existing SharePoint Server 2007 deployment by performing a search-first migration.
To perform such a migration, you deploy a new SharePoint Server 2010 farm and configure it for search. You reproduce the existing SharePoint Server 2007 farm search settings in the new farm, and then configure the SharePoint Server 2007 farm to forward search queries to the new farm. For this migration you don’t migrate content databases to the new SharePoint Server 2010 farm, for example, or configure any functionality in the new farm that isn't related to search.
After the search-first migration, your users can take advantage of SharePoint Server 2010 query features and search-results features right from your SharePoint Server 2007 farm! These new query features include prefix matching, Boolean query syntax, phonetic name matching, and query suggestions. Search-results features include results refinement, social tags integration, and relevance improvements. And your users continue to use the existing SharePoint Server 2007 deployment for SharePoint functionality other than search.
Your organization can complete the remainder of the product upgrade at any time that is convenient, but the enhancements that your users gain in search capability will probably give your business decision makers an incentive to fully adopt SharePoint Server 2010 as soon as possible!