Just to gather a more complete discription of the problem, have you tested this with different files, different fields and different fonts specified for the field?
Thank you for your post.
I am unable to replicate the problem on using FileMaker Pro 11.0v3 (and 11.0v4) under Mac OS X Lion. My default font is Helvetica 12 point. I get the cursor anywhere in a field (Browse) or a Text box (Layout). Let me know what I'm doing differently than you, and I'll try to replicate the problem.
Try creating a new Layout. Also, create a new database file. See if the problem is related to a specific Layout, Table or file.
The problem seems to be intermittent, and after speaking to people at DevCon, it seems the issue even occured under Snow Leopard.