is this a bad thing nowadays??? and if so who says so?

While there are some boards that explicitly prohibit it, it is in generally held as bad taste. Now, if 2 corps take on two other corps that may be different. In general, Megacorping is also on the level of cheating to most players today. Personally I think it is utterly lame to megacorp, but that is only my opinion. I would not play with people who megacorped on a server were it was not allowed / encouraged. Examples: Alienbase, Black Sun, TLTT, to name a few are servers were it is either forbidden (TLTT) or highly frowned upon.

Jhereg

PS: Megacorping is sharing resources, so a corp passing info I don't see as megacorping. A corp giving another corp a few planets and some cash to help them wage war, that is megacorping.

HUH? what is megacorping? Two corporations basically acting as one force?

Ovid962,
Yes.

One thing that Jhereg didn't really mention is that it isn't always considered bad taste if the total number of players in the two corps doesn't exceed the maximum total corp size. So if the max corp size is 6, and you have two corps of 3 working together as one corp, that USUALLY isn't a problem. (Usually it's better to just run a mixxed-corp.) When in doubt, check with the sysop first.