It works for me.
Please check that you have aspell (and the languages you want) installed in a right way, and that kdelibs use it.

I can confirm that it does not work for 64bit, but does work for 32bit. Are that your plattforms?

For me inline spell checking is not working (Kmail 1.13.3 on KDE 4.4.3, Archlinux 64bit). With Auto Spell Check enabled in the Composer Window (Rightclick anywhere in the message) misspelled words are NOT indicated with a red underline. In the Background Kmail recognizes that a word is misspelled because right clicking on the work offers you a list of corrections.  

Spell checking generally works if initiated from the Menu although I had to install the aspell-en dictionary which per default was present in the list of available dictionaries but not working. It then walks you through a spell check in a popup window which is less than optimal since the background window gets darkened (KWin) which makes things hard to read. 

>Please check that you have aspell (and the languages you want) installed in a
>right way, and that kdelibs use it.

Inline spell checking generally is working in Kate with red underlines but not in KMail so atm I assume the problem is with Composer


This bug report is old - Should I open another one maybe with screenshots and some more information? It would be really great to get this fixed - especially if you have to write in several languages a spell checker is indispensable for professional emails.

Me too :(. 
Exactly as described by Thomas wrt Kate, right-click corrections popup, and Menu activation all OK, but no underline. 
Also 64 bit as Christoph mentions
Kmail 1.13.6;  KDE 4.5.5; openSUSE 11.3 (x86_64)
Problem persists in fresh install of openSUSE 11.4

It seems to me that it's just the underline rendering that isn't working, as if you see a misspelled word and right-click it, you get the usual context menu with correction suggestions.

I'm on 64-bit KDE 4.6.1 and kmail 1.13.5.

Is by chance anyone with the problem running the nouveau driver?

I have 3 pretty much identical PCs (all arch64 with nvidia cards). My media PC nvidia-binary driver) where I never opened Kmail IS displaying the underline. My work PC (nouveau driver) where I never opened Kmail before is NOT displaying it   

My laptop displayed underlines for a while but then it stopped working again. This was still on 4.5 and the only thing I remember changing since then was moving from nvidia-binary to nouveau and upgrading to 4.6 (and deleting .kde4 in the process). 
Coincidence?

Driver seems far fetched but who knows. Is there maybe a funny method Composer uses to draw the spell checking? I noticed that the color of the underlining on my media pc is different compared between Kate and Composer. Kate is light red and Composer dark red.

@Thomas: Nice idea, but in my case it's happening when using the proprietary nvidia driver. :(

I am seeing this no-underscore problem with both the proprietary and nouveau drivers, on the same amd 64-bit box, on openSUSE 11.3 and 11.4 respectively

While looking for a workaround I discovered that if I copy text showing no underlines to clipboard, and open SpellCheck 2.1 plasmoid, the underscores display in a window which seems to work exactly as do Kate and Kmail(when working), popups and all. I suspect it is the same spellcheck component being used.

I am not sure where the widget came from, author's site http://emdek.cba.pl is in Polish, a show-stopper for me :(,  but I spot no link to SpellCheck. http://kde-look.org/content/show.php/Spell+Check?content=94113 is old. I will do some web searching.

Bug as described confirmed with Kubuntu 11.04, KDE 4.6.2, kmail 1.13.6 and the nvidia drivers.

The work around solution offered in the duplicate bug works: 
https://bugs.kde.org/show_bug.cgi?id=248381 

Apparently, this has been recently fixed, so closing as duplicate.

*** This bug has been marked as a duplicate of bug 247486 ***