Well without turning this into a bragging competition...

@thisandthathereandthere
I have (currently) 577 contacts listed in my contact list. This is after I ahve merged them (didn't take long at all) and this manages fine. Lots of my work contacts have multiple emails, Skype (some multiple ones), GTalk and several emails. Lots and lots of them have multiple phone numbers (cell and landline) and even a fair few with SIP as well.

I have Skype, GTalk, SIP, OVI and the others too registered and it all seems to work fine for me. (I am SO glad contact lists for the phone works in portrait though! LOL).

Basically a Little bit of attention and merge things properly and it all works well. You can always edit the nickname of IM contacts if they don't match to make the merging easier to handle (sort in abc order afterwards.).