This sounds like a simple question. I will say like many other developers/architects/consultants - "It depends..."

Bottom line AJAX is bad for SEO!
For publicly facing company websites, were SEO important, stick with the server side scripting such as ASP.NET, PHP, Ruby on Rails and others.
I am not mentioning static HTML pages here since medium and big size companies most likely will have data driven web site.

If you are building Line of Service business application AJAX will only make your application better.
Do not think twice learn it well and use it.

If you absolutely have to use AJAX follow "Unobtrusive JavaScript" pattern.
AJAX is great tool when used for proper application types.

Here is my prediction -> 3 years from now Search engines will learn to understand and properly index and rank RIA/AJAX/FLASH/Silverlight/Flex/Put your faviorite client side technology here. Until then ...

AJAX and SEO: How to have an SEO Friendly AJAX website using jquery
http://www.davidpirek.com/blog.aspx?n=AJAX-and-SEO:-How-to-have-an-SEO-Friendly-AJAX-website-using-jquery

http://www.seomoz.org/crawl-test

12 More SEO Tips for 2007 http://www.seochat.com/c/a/Search-Engine-Optimization-Help/12-More-SEO-Tips-for-2007/

SEO Myths  http://www.seochat.com/c/a/Search-Engine-Optimization-Help/SEO-Myths/3/
SEO for AJAX http://www.johnon.com/270/seo-for-ajax.html
AJAX, Web 2.0 and SEO http://www.hybrid6.com/webgeek/2007/01/ajax-web-20-and-seo.php
Web 2.0 Technologies and Search Visibility http://searchenginewatch.com/showPage.html?page=3624222
Unobtrusive JavaScript http://en.wikipedia.org/wiki/Unobtrusive_JavaScript
CSS, AJAX, Web 2.0 & Search Engines http://www.seroundtable.com/archives/006889.html
Search engine optimization http://en.wikipedia.org/wiki/Search_engine_optimization