I spend a lot of time answering emails from clients on this very subject. There's so much information about SEO online that it's easy for someone who spends just a couple of hours surfing to think that they've got a good grasp of how search engines work.
The problem is that the majority of the information and tools (such as spider simulators) are hopelessly out of date.
For anyone still wondering why metatags are suddenly no longer relied upon ....
Search engine spiders are constantly evolving - they are much more sophisticated than the ones a few years ago. It used to be that the spiders did look for metatags that would describe what the site was about.
Obviously that is very open to abuse - if you have a website that only sells red widgets, then what's to stop you putting "green widgets" in your metatags? People searching for green widgets would end up at your site even though you don't sell them!
The search engine market is fiercly competitive. All the big players - Google, MSN, Yahoo, Alltheweb etc want the biggest share of the market, and the only way they are going to get that is by giving visitors accurate and relevant search results.
Because of that, spiders have now concentrate on looking at sites the way visitors see them. If you have lots of good content on your site, spiders will have no problem indexing that and passing that onto searchers who are interested in what you have.
Most attempts to manipulate search engine results now fail - they may work for a very short time, but the search engines soon pick up on it. Search engines will index sites they way they want to, not the way you want them to.
If you keep that in mind, and make sure that your site has a lot of keyword rich content (not in a spammy kind of way) then you won't go far wrong!
Same goes for a lot of other advice given on the web. A lot of these spider simulators that you see online which you can use to check your site reckon that they can't index sites with dynamic content - osCommerce sites are a good example of that. We've got clients whos osC sites are getting spidered daily and have literally every word of every page of every product indexed and listed. The fact is that the simulator people aren't going to put as much time and money into developing their spiders as the real search engines do.
Of course, all that's just my opinion on it - the real joy/pain of SEO and website marketing is that everyone has their own ideas as to what doe/doesn't work and really there's no right or wrong way of doing things ...