Thanks for your bug report. Unfortunately I have not been able to reproduce this issue. I use the full screen feature in mIRC all the time on Windows 7. When mIRC is set to full screen, Windows hides the task bar. Windows should do this automatically for any application that is set to full screen.
I do not run windows 7 (still on xp) but on XP you can lock the taskbar so it does not hide, if this is possible on windows 7 (again i dont run it) it might be a reason..