If an application doesn't show up in the Nokia 770's task navigator, the reason is usually it's missing or wrong StartupWMClass setting in the .desktop file, usually found somewhere in

/var/lib/install/etc/others-menu/extra_applications/<application>.desktop

Luckily, finding the proper setting is usually rather easy:

• Launch the same application on some Linux machine (not the Nokia 770 itself!)
• run the xprop command (which is available on probably all Linux distributions, Debian ships it in the xbase-clients package; unfortunately it is missing on the Nokia tablet itself) and look for the line containing WM_CLASS(STRING)
• use the exact string that is written right after the equal sign for the StartupWMClass setting

E.g. for Gnumeric (thanks to Eduardo de Barros Lima for the Maemo port!) xprop returned

WM_CLASS(STRING) = Gnumeric

hence setting

StartupWMClass=Gnumeric

in gnumeric.desktop solved the problem of Gnumeric not showing up in the task navigator.