Hi Pettan,
I waited long enough for someone else to answer this one but I couldn't wait any longer. Be prepared to duck P'Man. I'm armed with the rule book of doom and I'm not afraid to use it!
Got this from The EPICenter (http://netepic.org/NETEPIC/) under Playaids there is a file for FAQ that covers this question. It states...
Remember infantry needs to be directly attacked to engage flyers and then only with a -2 (To Hit) penalty. This includes HQ's that are infantry units as well as Psykers that are infantry units. This gives a slight edge to Orks who have weirdboys in vehicles (Weirdboy Tower) and thus could function as a poor mans AA guns. Normally most Psykers would do poorly against flyers.
It also previously states...
Only an attack that has a direct and immediate effect, like a bolt of energy (Physical Psychic attacks) like a Weirdboy (Tower) affects flyers. Effects that have a template effect or require that the victims mind be contacted (like Mind Blast) DO NOT WORK. Basically if it works like a 'shot' than it effects a flyer. If it doesn't work like a standard 'shot' then it doesn't work. This is to avoid cheesy play like casting vortex/tempest/mind blast and other such attacks on flyers. Flyers would easily by pass such attacks due to their sheer speed and too little time in one place.
And if you manage to figure out what all that means let me know...
I figure your Weirdboy Tower can fire at flyers even if it is not the target of the flyers attack. However due to the nature of the psychic attack being 'Physical' it is treated as a standard ranged 'shot' and it will suffer the -2 To Hit penalty.
Now if you don't mind I'm just off to soak my head after warping my mind around that one.
). Given the number of awesome special units the Orks have to choose from I rarely see my Orky opponents take a plain Weirdboy actually.