As far as having to "file" taxes... I think we should definitely leave that out. By that time, technology will be advanced to the point that the gov't will know most of the info about law-abiding citizens. Thus, they can tax people correctly, according to whatever rules are in place, without any user intervention. In any event, if we add a bunch of bookkeeping to a player's responsibilities, people will get bored / frustrated / torqued off, and leave. Not good. At most, we should simply send them the bill or something, and have them click a button to pay it. This gives people the option of not paying.
Also, instead of income tax, we could have a sales tax. Texas, for instance, doesn't have a state income tax; rather, we do sales taxes (and property taxes and a bunch of others at the varying levels of gov't that I haven't taken the time to research yet...). So we could have things in the game have an additional tax on them (that should be clearly evident when purchasing something - that way, we don't get someone going "But it says $80k, and I have $80k?!"). Or, we could do property taxes - you can do the real-estate thing, but you have to be able to pay the taxes on the land you own. Or docking fees. These ideas sound good.
Or, we could have the Terrans do an income tax, and the League do property or other kinds of taxes. Or vice versa, or some combination. Makes working with one group or another more interesting.
I'm not sure taxes are such a good idea, though. On the one hand, it does add to the realism of the game - we have governments, and those governments must be sustained. However, I'm not sure people will enjoy playing the game if there's an added burden of paying taxes, regardless of the type.
If we do end up doing taxes, it MUST be for a purpose other than "realism" - the money must actually go somewhere, to someone, to do something that the players can see. Otherwise, people will resent paying them (much like many do in real life...). So before we decide HOW to collect taxes, we must decide WHAT that money will be used for. And if the ideas don't knock our socks off, I think we should consider dropping the tax idea. I don't think we should just create "money sinks." We should do something that the players will find worthwhile.
~Brian