i've done - twice.
but the problems you will face are 3-fold:
#1 voiding your warranty. different people will have different perspectives on this. personally i couldn't give a damn. i always break my warranties and in recent memory i can't think of a time where i wish i had not. with all the people advocating AppleCare i doubt i get much support, but i'm a hardware geek so warranty be damned!! (i overclock and mod too
)
#2 getting the drive you want. laptop drives can be tricky to acquire. i bought mine and used to sell them too, from here in tokyo. ebay is your best bet i think. i'll leave you to decide which drive you want. it would be better to choose models that apple uses however, because iTunes and DiscBurner need 'hacks' to use other drives.
#3 taking the iBook apart is, well... a bit of an ordeal. i've done it countless times now. though i have wound up with a few extra screws.
Google.com is your friend.
xlr8yourmac.com is a great resource too.