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They provide a WAP (Wireless Application Protocol) Gateway to access their search results and convert the resulting sites to wml (or equivalent) on the fly. That lets browsers on Mobile Phones (note the UP browser ... Urban Planet) access whatever content they find via their google search, even sites which would not normally be browseable.
To learn more, try searching google.com for WAP sites.
For more about Google's implementation of it, try searching for google WAP sites.