[UPDATE: del.icio.us tag search field test underway]
By exploiting the same mechanism that google bombs rely on, you can generate your own page sets and search only those pages with Google. A minor example.
Since text in links leading to pages count as text on a page, you can simply doctor link text to generate your search subset by including a special token like 3f4h9hgfh98fh9p348gh .
Now searches for e.g. hosting can be bound to the pages I link to by requiring my link identifier token in searches. By augmenting the scheme a little (e.g. by agreeing on a naming convention along the lines of gtag_tagging for the google tag 'tagging') we could have google search of our tag database quite easily. Obviously this is just a poor mans replacement for being able to limit search by forward links.
Desktop search engines are nice, and tagging is all the rage, but tagging still isn't all that, because del.icio.us isn't also a search engine.
Obviously what we would really like is for a major search engine to just embrace tags. This of course would lead to immediate scandal, because everybody would be screaming "Autolink Scandal!" 2 minutes after launch. After all, it's not really that nice for a content provider that I get to augment his content and tagging does exactly that. (Incidently, have the political defamation tags - a la Google Bomb - begun to appear?)
If google tagging were to become widespread, Google should embrace it and simply start supporting real tags through e.g. rel attributes (like technorati) and Tim Berners Lee should declare victory for the semantic web. There is no important distinction between RDF and tagging at present. Later it might be nice to turn tags into full blown RDF assertions to qualify them. Berners-Lee might not see this is semantic victory though, because the ideas on semantic assertions based on RDF metadata will not come to pass because of tagging.