Little by little, Google has been improving search in Gmail. Back in May, Google made improvements to autocomplete predictions in Gmail, so that they would take into account the actual content of emails. About a month ago, Google added support for autocomplete predictions in the From: and To: fields, so you can find messages exchanged with specific people more easily.
According to a report from Google Operating System, Gmail recently began letting users search inside attachments. This includes support for PDFs, DOCs, and PPTs. "To find messages that have attachments, search for has:attachment and add some keywords to your query," suggests Alex Chitu. "To restrict your search to PDF files, search for has:attachment filename:pdf."
One reader commented that it was not working on Google Apps, but was for personal Gmail.
Google did recently launch Gmail Log Search for Google Apps admins.
Of course, the most noteworthy feature Google has added to Gmail, in terms of search, is the addition of Gmail results to the Google web search results pages. Granted, this is only in limited trial at this point, it's an interesting feature that puts email into the greater search experience, and could even have some ramifications for email marketing.