Whenever I run Messaging for a while it does something to my GPRS connection rendering it unusable. It is marked by the icon either as "on hold" or "available but inactive" and no application can use it. Switching Messaging to offline mode and back to online doesn't help (it doesn't go 'online' anymore), neither switching it off altogether. Only restarting the phone helps 
Is it a bug of Messaging on 5800? Frankly my phone is NOT listed as supported here (I know it is on email.nokia.com):
compatible-devices
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alkmeneDzid
5800