Regarding your requirements for voice communication I would recommend either Alditalk (€ 0,03/min to other Alditalk subscribers, € 0,12/min to North America and domestic networks) or Fonic (€ 0,09/min to other Fonic subscribers, North America and domestic networks). Both SIMs are easily to buy, as Alditalk is sold at omnipresent Aldi supermarkets and Fonic at Lidl, Real, Müller and dm supermarkets, which are even wider spread.
Another option would be
solomo pro (if you order through
this link, you'll get € 5 of extra credit and so will I) with € 0,05/min among solomo customers, € 0,09/min to North America and € 0,10/min to domestic networks with the great advantage of billing by seconds after the first minute, whereas the afore-mentioned round up call time to full minutes. The only downside of solomo is, that SIMs are only sold online and can also be recharged online by creditcard only.
Which of those is the best for you depends on distribution of destinations called.
The G1 can be tethered to a computer and so be used as a modem, but afaik the appropriate application has been pulled from the Android store and can now only be installed by rooting (hacking) your phone. However extensive modem-usage will wear out your G1's battery soon and so it may be reasonable to get a USB-modem instead.
When it comes to a modem, that will work in the US later, you should be aware of the different frequency bands used for 3G in Europe and the US:
Whereas European 3G-networks operate at 2100 MHz (and some outside of Germany also at 900 MHz), American 3G networks use 850 and 1900 MHz (AT&T) and 1700 and 2100 MHz (T-Mobile), whereby T-Mobile uses a combination of 1700 and 2100 MHz (so-called AWS), which is incompatible with the European 2100 MHz-band.
As there's no device available in Europe, that supports T-Mobile's 1700/2100 MHz-band, you could only get a device that will work in Europe and on AT&T's 3G network, but not on any other US-network (as the rest are not using UMTS-technology). So in order to get a device suitable for Germany and AT&T you would need a device supporting 850, 1900 and 2100 MHz. The only reasonably priced device meeting this requirement would be a Huawei E160.
Actually the Huawei E160 is sold by several providers (Tchibo, Fonic, Vodafone, Aldi) starting from € 30, but they all have different designations and I fear some or all are a trimmed version called E160G, which lacks support for UMTS 850 and 1900 MHz and so won't even work on AT&T's 3G network. I'll try to find out on German forums, if the E160 is available somewhere and let you know soon.
As you already found out, Tchibo is perhaps the best choice for internet access. If you want even better quality (better 3G coverage and even higher bandwidth) Vodafone is interesting, as they have a summer promotion offering 7 days of internet access for € 10. However Vodafone's so-called "WebSessions" will be terminated after reaching 1GB of data volume. That could easily happen sooner than 7 days and so you could need more than 3 WebSessions (each costing € 10) to stay online for a month.