Neil Skywalker said:
A possible reply will be that more sites use the phrase "not so lonely planet". i did a google search on it.
http://www.google.com.py/search?aq=f&sourceid=chrome&ie=UTF-8&q=not+so+lonely+planet
This doesn't matter unless you're going to argue their trademark is already diluted, but I wouldn't rely on it.
Are they gonna sue any using the words Lonely planet ?
Depends on context (see trademarks). For those which are relevant they actually have to as otherwise they may eventually lose the trademark protection.
The most important question for you, however, should be if you're going to get sued, and what would it mean for you.
Putting (R) after "lonely planet" and making a footnote saying "Lonely Planet is a registered trademark of Lonely Planet Publications Pty Limited" may be enough in your case, but you need to talk to a lawyer since you already received the infringement notice.