The answer below was correct from 2009 to 2013, but an update was made after the blog post I cited that made it so tags in search terms won't be converted unless all words in the search can be converted.
Source: Nick Craver's answer here, and @V2Blast's comment below.
Only the top 60 tags on the site you're searching will automatically be converted. support and stackoverflow are currently the #3 and #6 tags respectively on Meta, so they get converted when you enter them as search terms. You can circumvent this conversion by putting your search term in quotes. Example: "support" or "stackoverflow".
Any individual search terms which map directly to the top 40 tags will be auto-converted to tag searches. So if you enter
it will convert to
automagically on your behalf.
Source: Stack Overflow Search – Now 61% Less Crappy