I am not 100% sure, and you should back up your data.
but back up is must if you are installing from USB or cd/dvd.
directly upgrading from servers is much more safer, and most probably will not lose data, i am not sure about added applications.
as you can see it
https://help.ubuntu.com/community/NattyUpgradesNetwork Upgrade is mentioned as recommended. (and not dvd/usb) when you upgrade 11.04 to 11.10make sure that your 11.04 is completely updated.network is recommended only when you have a complete successive version installed, which applies to your case. so not an issue.keep that in mind.sometimes applying old data to upgraded version can cause errors.To cut a long story short.(you can download 11.10 ISO to usb/dvd,
for other uses in future, but dont install from them)
go for a network upgrade
(instructions given here, ithink will be the same for 11.04to 11.10)
take back up for safety, but upgrade from network and not usb/cd/dvd.