I am sure you have seen this, where you come from a Google search and land on a page and you notice the the keywords you searched on are highlighted within the context of that page you land on. If you haven't try this search phrase, click on this search phrase then click on the first result, it should take you to DigitalPoint's forum and highlight the words.
Now what if you used a variation of this and instead of just highlighting the keywords, you insert them into the headline of the page. Why would I want to do that? Well, there have been tests done that show when you have the keywords in the landing page, it will ultimately lead to a high conversion rate. If think deeper about this and maybe the dynamic insertion of keywords into the page will lower this conversion rate in the long term. Anyway, it can be done.
There are two issues that come to mind:
(1) What if the search query is not in proper grammatical sense? Your page will look weird. Misspellings, offensive words and so on.
(2) The second issue is that the page is, in a sense, cloaked. Because the search engines can not retrieve the same page, exactly with the same context, as the user.
These issues are now being discussed in a Search Engine Watch thread.