No. Linux isn't run on any accounts anyhow. The accounts aren't even stored on your ps3, just the login id connected to them. And those are both part of the main XMB partition. The 10gb OtherOS partition is completely separate from that, so when you run linux, you won't be running under any login ID at all, except for the username you create for linux.
All your data will be deleted, yes - but that doesn't mean you have to make backups of everything. Only what you can't replace by downloading. Usually you can fit all your game data and saves onto a flash drive and put them on a pc. It doesn't take much space to back up videos, music and photos either. Demos you can just re-download. Downloadable full games is the one where you'll have to decide whether to take the trouble to back them up, or to use up one of your 5 downloads after you install linux.
Reformatting to create the otheros partition wll take about an hour, and the install for most distros takes about half an hour. If I remember right, the XMB stays installed, that is it doesn't have to download again after formatting.
Having to back up all your stuff is a nuisance, but it only has to be done once.
Once you have installed otheros, you might want to add a usb external hard drive, particularly if you're using Yellow Dog. YDL is way too big of a distro to just have 10gb of disc space.
I also strongly recommend immediately downloading the mplayer plugin for Firefox, then adding Greasemonkey to firefox and installing scripts that make Firefox use mplayer. This allows you to use mplayer for flash video, it is much better than Adobe's crap linux flash player. Also you will initially find that dvd's and other stuff won't play at all; you need to install non-free codecs and restricted packages, this is because dvd's use CSS encryption and dvd players are supposed to pay to be compatible. You can find the packages fairly easily in the repos though. If you are using a red hat-based distro like ydl, and can't find them, look for them at www.easyubuntu.com, download them without installing, then download the "alien" package to convert the debian packages to red hat, then install.
Once you get it going, you'll find your ps3 under linux is better at watching Youtube than the XMB is. Particularly since with mplayer, you can download and keep anything there.