I am doing this out of the goodness of my heart. And also because I enjoy complaining about it but people have stopped listening to me.
So, you're going to Germany. Great! Oh but you're worried about not having a mobile phone? No problem! SIM cards are easy to purchase and you can probably google around and figure out how to unlock your phone. Unless you have Sprint or Verizon, in which case go hide under a rock until you feel ready to join the rest of the modern world on GSM. Do not just "use" your regular US phone abroad (it will probably work), unless 1) you are very rich and 2) looking at ridiculous bills doesn't give you a splitting headache. If you're fine with that, then go right ahead.
If you hate actually talking, then clearly a phone is not enough and you also need a data plan. I recommend blau.de. Setup is a little tricky though, as all instructions and messages and the entire website are in German.
You can purchase a blau.de SIM card at places that sell ePlus once you're in Germany, which is nice. It's about 20 euros, but that comes with 20 euros of credit. The shop keeper can set it up for you, but he will need an address in Germany for the form.
Once set up, you basically just need the first PIN to activate the SIM card and be able to make calls. You can turn off the need to enter it over and over in your phone. At the time of this post, blau.de had 1 GB of data in a month for 9.90 euros, a pretty good deal. I even used it to tether to my laptop and I didn't use anywhere near the whole GB in two weeks. You order it by calling 1155, and following the menus: press 8, press 1, press 2, and then (THIS IS IMPORTANT) pressing 1 again to confirm. Do not use data until you receive TWO text messages (the first will come immediately and say that you ordered it, the second will come within 2 hours and say that it's ready).
To enable data, set up a new APN. APN internet.eplus.de, username eplus password eplus. Especially if you are using the G1 (google phone), DO NOT DO THIS UNTIL YOU GET THE SECOND TEXT MESSAGE. Otherwise your stupid phone will churn through your credit eating data while it downloads a whole new android operating system. Sigh.
Notice that I haven't told you how to reload your credit. At the time of this posting, they wouldn't take credit cards with an American address. I managed to convince them to do it once but I still had to call customer service number (which costs like 0,30 euro per minute) every time I wanted to recharge, and my friend couldn't do it at all. If you have a credit card attached to a German address, you can do all of this through the website. Run it through google translate.
Good luck and have a great time in Germany!