Trademarked words are filed in all capital letters so an upper or lower case "a" does not make a difference. More generally, any variations of a mark that would be confused with the original would be trademark infringement so adding/changing a letter or two or punctuation is probably not enough of a difference in most cases.
Alain's answer is correct, but you need to watch out for other trademarks as well. For example, Lucasfilms has a trademark on "Droid". You may not infringe the Android mark, but you may infringe the Droid mark.
Note that Motorola licensed the Droid trademark from Lucasfilms for its Droid phones. Here is an article about it.