Metatags ARE a fundamental aspect of search engine optimization. Still! (even in 2009.)

There are three primary metatags that we are concerned with-as far as SEO goes- the keyword metatag, the title metatag, and the description metatag. Notice these are three separate things. They serve different functions. They also have different levels of importance to search engine optimization.
In level of importance:
1) The Title Tag
2) The Description Tag
3) The Keywords Tag

The Keywords metatag is a distant third.

The Title MetaTag
If you look across the top of your browser, you will see the Title Metatags I have filled out for this page. On the homepage of RoseOspreyMarketing.com the title tag reads:
Search Engine Optimization, Pay Per Click, Adwords Consultant
These are the primary keywords for the content on this page. This is where you put your best keywords, as this has the most bearing on SEO. It is okay to separate them with a comma, or a |. The goal is to get as many important words in there as possible without overflowing off the screen.
Each page on your site should have unique title tags. Also, you do not need to put your domain or brand name in your title tag. That is wasting valuable real estate.

If you are new to SEO, and you want to fill in your Title tags, how do you choose what words to put into the title metatag field: Look at the page you are editing and look for themes, or categories or products that are unique to this page. Stick those in the title tag.
Or, if there is a special word you’d like to rank for, and you’d like this page to be the landing page, put that special keyword in the title metatag on your page.

The Description MetaTag
The description metatag is used by certain search engines (like google and yahoo) to provide a “blurb” about your site on the search results page. Sometime search engines will show you an excerpt of text from the website, but often it will show the description metatag.
This is an opportunity for the savvy marketer to write some keyword rich copy that encourages searchers to click-through to your website. So, the description metatag will never be a list of keywords separated by commas. It has to actually be a sentence.
Other considerations:
Every page on your site needs to have a unique description metatag.
It should be 2-4 sentences. Too long or too short is not optimal.
The content of the description metatag should relate to the content of the page.
Add as many keywords in the copy as possible- but keep it readable.

The Keyword Metatag
The keywords metatag is a distant third in terms of importance to SEO. Many top SEOs ignore the keyword tag because they feel it is not cost effective, from a time-management perspective, to focus here. Time would be better spent on link building or developing content.
I would not recommend spending much time researching keywords for the metatags, but I do believe they should be populated with words relative to the content on the page.
Another good use for the keyword metatags is to put frequent misspellings of your topic in list form.
If your page is on leveling in Warcraft (the videogame) a good word for the keywords metatag might be “worcraft”(sic). In this way the search engines can know you have that content, but you don’t have to show your customers ugly misspellings.