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  lasmerge:

  reads multiple LIDAR data files in the LAS format and merges
  them into a single file. The filenames can either be provided
  one by one or in form of a text file.

  All the header information of the first file provided is used
  including variable and user_defined headers. but some records
  are updated by integrating the corresponding information from
  other headers. these are:

    * number_of_point_records
    * number_of_points_by_return[5]
    * max_x, min_x, max_y, min_y, max_z, and min_z

  In addition x_scale_factor, y_scale_factor, z_scale_factor may
  need to be increased to accommodate a possibly larger bounding
  box. The user can also set those in the command line with

       -scale 0.01     or     -scale_xyz  0.01 0.01 0.001

  Similarly a new offset can be specified

       -xyz_offset 63025000 483450000 0

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example usage:

>> lasmerge -i in1.las -i in2.las -i in3.las -o out.las

merge the three inX.las files into one out.las file. 

>> lasmerge -i las_file_list.txt -o out.las

merges all LAS files listed in the text file into one out.las file

>> lasmerge -i las_file_list.txt -o out.las -scale 0.01 -verbose

merges the file and stores the LIDAR points with 0.01 accuracy 
in x y and z and prints out control information

>> lasmerge -i las_file_list.txt -o out.las -scale_xyz 0.01 0.01 0.001

same but with a different accuracy for z

for more info:

C:\lastools\bin>lasmerge -h
usage:
lasmerge -i lasfiles.txt -o out.las
lasmerge -i file1.las -i file2.las -i file3.las -o out.las
lasmerge -i file1.las -i file2.las -olas > out.las
lasmerge -i lasfiles.txt -scale 0.01 -verbose -o out.las
lasmerge -h
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if you find bugs let me (isenburg@cs.unc.edu) know.