I gave my friend btc (ie sent it to their address) to trade together with their btc last week. I'm suddenly worried about taxes (in the US). If right now, they give me back exactly as much btc as I gave them, are there tax ramifications? Can I sell the btc after > a year and pay the 15% capital gains tax or whatever, or is there some short-term hold complication, and do I have to pay more?
They will most likely see any BTC credit in your account as "income" and you are taxable for it, unless you can prove it otherwise. This is a matter of when the IRS will start cracking down on these exchanges and request them to provide a 1099 for every user. I do not do trading via fiat because of this. The end-game is to acquire more Bitcoins, not fiat.
When its time to sell your Bitcoin for fiat, you won't have to because everyone will be willing to take it. -- See this: http://imgur.com/gE8hDnY
Not a tax expert and this is not tax advice, just my understanding:
A taxable event does not occur when the bitcoin comes into your account, any more than it would if you were depositing US dollars into a regular bank account.
The taxable event occurs when you dispose of the bitcoin for a gain. If you gave your friend bitcoin as a gift, for example, I don't think there was a gain. (Although there are limits on how much you can gift.)
If you sold it to him, then there was probably a gain (the difference between what you sold it for and what you paid for it). You only pay taxes on the gain. Re-purchasing the bitcoin would have no effect.
As far as I know, in the US, the gain is not considered a capital gain, its considered profit on the sale of property, so the rate is different (higher).
This assumes you already paid taxes on the money you used to acquire the bitcoin, but that is not the question you asked.