re-entrancy question

[apocalypznow]

I have a program that instantiates some objects and runs a main loop. 
Before the main loop starts, I create a thread that listens to TCP 
connections on a port.  If any connections are made, and depending on 
the data, I call methods on some of those objects.

I am worried that calling methods on objects from outside the main loop, 
but that are also operated on in the main loop, could cause problems.

My questions:
1) is this safe?
2) what kind of problems could occur?
3) how do I safely handle this?