Twitter has a new section on their website offering information on how businesses are using twitter for marketing and customer service.
This Twitter & Social Media Marketing Guide includes recommended books, Twitter definitions, and case studies of businesses using Twitter for marketing:
When people working in the Empire State Building twittered that they were craving ice cream delivery, New York local chain Tasti D Lite was there to listen and meet their need. When electronics buyers look for good deals, the Dell Outlet Twitter account helps them save money with exclusive coupons. …

Google Wave Marketing? Wait a Minute, What is Google Wave?
Coming late 2009, Google Wave is a mashup of communications tools provided by Google to make it easier for people to work and communicate online. (Read more about Google Wave)
 
What Could Google Wave Change for Internet Marketing and Social Media Marketers? 

Google Wave could further fragment online audiences and limit the number and growth of today’s large social networks.
Dave Coursey of PC World says “Google Wave as a mash-up of chat, e-mail, and document sharing, it’s really something else: Google Wave could be …

 Apple’s new speedier, feature-rich  iPhone 3GS will provide exciting new marketing opportunities for Internet marketers.

Rich Media: Faster downloads, greater memory and processing speed will enable increased use of rich media advertising to iPhone users. Since the new iPhone announcement in March, some agencies have already experienced a 20% increase in iPhone advertising inquiries.
Geo-targeted Ads: the new Safari browser will have location-based technology, allowing marketers to develop geo-targeted ads for iPhone users (assuming the user gives permission to the website to see his/her location). Marketers will be able to serve ads …

Six Strategic Uses of Twitter in Marketing & PR
Growing numbers of individuals, small businesses, corporations and non profit organizations are jumping onto the Twitter bandwagon. Those who will find the most success will take the time to consider how they can use Twitter to their strategic advantage. Here’s a few suggestions how to make the most of Twitter for your marketing & PR:
Marketing & Advertising
Follow customers & future potential customers. Integrate your online and offline marketing &
advertising messages with your Tweets. Provide links to promotions, microsites, videos,
advertisements.
Reputation Management
Follow experts, industry leaders, news …

Twitter Marketing 101: Ten Ways to use Twitter for Marketing & PR for your Company or Personal Brand

Get a Twitter Account. If you don’t have one already, visit Twitter.com and sign up for a free account. Think carefully about what your twitter account will be called as you can’t change it later and it should clearly represent your company or brand. 
Create a Twitter Marketing Strategy.  While most people just start randomly posting whatever comes to mind, you’ll be much more successful if you first sit down and write out your …

Resources for using Twitter as a marketing, PR, networking and communications tool

Rafe Needleman: Newbies Guide to Twitter
Caroline Middlebrook: The Big Juicy Twitter Guide
Jeremiah Owyang – What The Web Strategist Should Know About Twitter
B.L. Ochman’s What’s Next Blog: How to Write Kickass Twitter Posts
PR Squared: Using Twitter To Create & Inform Communities
WebProNews: Microblogging What’s it Good For?
University of Maryland, Baltimore County: Why We Twitter: Understanding Microblogging Usage and Communities
Red66 – Using Twitter as a Marketing Tool
Eleven Marketing- Add Twitter to Your Internet Marketing Toolbox
Twitter Marketing Agency …