I was wondering, is there a problem with being a member of both the Mages Guild and the Dark Brotherhood ?

no, but there may be a problem with theives guild and mages guild

Nope.
I know what you're thinking.......but the staff stays there even when Traven.......

I just relised this isn't a spoiler section,so i'll stop.

And boonesim,you can become a master of every guild you wish at the same time.

Yes, they've made it so it doesnt conflict mostly...

But what if youve already pocketed the staff hypno?

EDIT:Spoiler removed

I already answered his question in the form of a PM so that I wouldn't be posting any spoilers

Thank you all for the responses. I was afraid that when I join the Dark Brotherhood that I would not be allowed to enter the Mages Guild to use the enchanting alter.

There is a part in the thieves guild with a wizard tower. You kill the enemies but the one who doesnt attack you dont kill. You also cant search any chests there. So it kills the point of looting.

hmm he attacked me on sight.... i wonder what i did to piss him off

Heh, I remember panicking when I had gotten rid of Traven's staff and got the Thieve's Guild quest.... and the relief I had when they put the correct staff in *my* quarters after! (Thank heaven - wasn't sure what I did with Travens, although I think I sold it to my usual person in Merchant Quarter and her inventory loses the best items after a few days).