The "Things I've Been Asked Lately" series of posts (TIBAL) is exactly what it sounds like. People ask me questions in real life, on the blog, or on the forums I follow and I endeavor to do my best to answer them.
Can I Use Brand Names in My Writing?
No, Wal-Mart will not sue you. Coca-Cola will not sue you. Neither with 3M sue you for having a character who covers the walls of her entire apartment with Post-it notes. (Yes, the term "Post-it" is a registered trademark.)
Your characters can go Rollerblading instead of inline skating. They can Xerox files instead of copying them. They can even reach for a Kleenex instead of a tissue when their lovers are hit by cars the day they were supposed to run away with each other. (These are all trademarks that are likely to slip into common usage in the next few decades.) And they can drink all the Starbucks their caffeinated little bodies can hold. (Most likely not going to slip into common usage but the point is that it doesn't matter.)
There are no legal ramifications of adding these things into your writing. There are, however, craft ramifications: Consider whether using a product brand name will date your writing.
Maybe dating your writing is what you want -- you know you're not in the twenty-first century when your character reaches for an ice cold Nehi** -- and those sorts of details add authority to your voice and credibility to your setting.
But perhaps your writing loses out for including something that's trendy at the moment. I snicker when I read fiction that is supposed to be "contemporary" but the characters carry PDAs -- the term may have been hot when the novel was drafted, but no one still carries a PDA that isn't also a smart-phone ... and an mp3 player ... and a GPS device and ... you get the picture.
Is it worth having your characters carry cell phones? Yes. consider that the cell phone is now ubiquitous in American culture. You'd almost need to explain why your character doesn't carry a phone if you set the story in the present day.
What it comes down to is two things. One: do you want to "date" your writing to a certain time period, or do you want it to attempt at being "timeless" and ubiquitous? If you have no doubts that it is of a certain time period/year, go for those specifics that have change or will change. If you want to create a false sense of it-could-be-set-anytime-recently then go easy on the brand specifics.
Two: don't over do it. Examples: Curious, Rachel decided to turn on her iBook open Firefox and Google it. Blah. Brand name overload. It wreaks the same as someone who always has to name drop. (Do you like my purse? It's Kate Spade.) Choose not to over do the brand name dropping: Curious, Rachel decided to do some online research.
** Ten points to the first person to correctly identify TV's favorite lover of orange Nehi and the show that featured said character.