Is it safe to erase "SDMHelpData" folder in user/library/preferences ?

Why do you want to do this? It evidently has something to do with Apple's Help system and is new in Lion. There is one multi-MB file that gets constantly changed or time-stamped and then backed up to Time Machine drives, which is annoying some people as it is a giant waste of space. If that is your case you can just exclude it from being backed up.

Francine

Yes it is safe to delete, but it will be rebuilt once you use Help. I found the following (translated into English):

The folder "SDMHelpData" contains a linguistic analysis of all words of all help manuals of all the programs you have installed, as well as information about your personal search behavior. It is used to make proposals for the search when you are working with search in the help. If the folder is only 300 MB in size, that will ever happen. With intensive use of Lion comes together quickly and over 1 GB.

Delete does not work, because the folder will be rebuilt as soon as you use the help. Exclude Time Machine should be no problem, but then the whole folder, not the file lsm_map.