Houston,
there is a slice function in the "save for web" option, but using that option you can't save the files as bmp (only as jpg, gif or png).
Another method sounds complicated at first, but once you're getting used to it, it's quite simple:
In this example, you'll make 9 slices of your bitmap (like a tic-tac-toe-game).
choose the rectangular marquee and select the upper third of your bitmap.
Select IMAGE / CROP.
Draw another rectangle for the first third.
Select IMAGE / CROP.
You now have the the upper left slice. Save this picture as whatevernameyouwant.bmp
Open the HISTORY window and delete the last entry (Should be crop)
Invert the selection and CROP.
Draw a marquee for the middle button then CROP.
Save as bitmap.
Again in HISTORY delete the last entry.
Inverse Selection - CROP
Save as bitmap.
Now you have the upper third of your picture sliced intro 3 bitmaps.
Open the HISTORY-window and delete everything except your first marquee (with which you selected the upper third).
Inverse the selection and CROP again.
Draw a rectangular marquee to select the 2nd third of your picture and CROP.
By repeating the steps, you should get nices slices of your bitmap. Due to the option "inverse selection" all slices should fit seamlessly together.
Like I said, this sounds more complicated than it really is, just try it step by step.
If I failed to explain, you can send me the file to be sliced 
André
