I am having the same issue now.

Battery ran out then phone switched off> Plugged in the charger> Removed the battery to turn the device on again> Connected to wi-fi.
"Poof" Nokia Messaging gone from list of service providers.
On t-mobile USA

Any ideas please?

Looks like there is a bug out there for it (sort of):

https://bugs.maemo.org/show_bug.cgi?id=7286

I ddi not reflash my device but it appears even that does not work.

Any updates? Would really like Nokia Messaging Push email solution back on my N900.

*bump*
I used Nokia Software updater to reinstall the firmware but still no Nokia Messaging available under providers.
Is using the software updater the same as reflashing?

i had the same problem - Nokia Messaging account would suddenly just disappear. i had to reflash using flasher.

well i have the same issue...no nokia messaging under service providers list. Please if anyone knows a solution let us know...and i dont want to flash my device and go through the entire routine of installing apps:P

Any other solutions anyone ?

i'm having the same problem-- no nokia messaging option in the list. i thought it was just because i had the region set to USA, but i tried other regions and no joy.

i had to trick the nokia messaging website by telling it that i had an n95 when i signed up, because it wouldn't let me sign up for an account when i said i had an n900. so i don't know if that has anything to do with it.

this is definitely a major problem for anyone who wants to use this thing like they would their blackberries....

This is definitely a major problem and no one seems to have answers, i've searched high and low and still no answers...submitted bug report friday no responce

Maybe we can get the settings and add it under "Other"

Please check that your rootfs is not full. Nokia Messaging is not available as an account type if it is.