I looked over this thread, but no one seems to have mentioned my favorite way of getting LW to run as an ultrapeer, so I thought I'd outline it here for whoever might find it useful.

First, exit LimeWire if it's running and check your taskbar, task manager, etc. to make sure the program is completely shut down and not just hiding. Next, find your LimeWire config file and open it in your favorite text editor; on Windows 7 machines, this will be at "C:\Users\{you}\AppData\Roaming\LimeWire\limewire.props". In this file, locate the lines starting with "DOWNTIME_HISTORY=" and "UPTIME_HISTORY=". (One way LimeWire decides if you're worthy of being an ultrapeer - possibly the only way - is by saving stats on past program performance and examining them each time it starts up. Ostensibly, ultrapeers should be on reliable machines, with reliable Internet connections.)

If your machine isn't trying to be an ultrapeer, chances are you have a lot of small (three- or four-digit) numbers in one or both of these lines, which will happen if you don't keep your PC running for long periods of time, or if you frequently run LimeWire in short bursts, say, to look for and download one file at a time. To increase your chances of LimeWire starting up in ultrapeer mode, add digits or remove semicolons in these lines to make every number listed at least five digits long - and, for good measure, find the line starting with "AVERAGE_UPTIME=" (this is usually the last line in the file) and delete it to force LimeWire to recalculate this number.

In my experience, every time I've done this, LimeWire has gone directly into ultrapeer mode at the next run. YMMV.