I'm going to India and I'm planning to take a couple of electronic devices with me that are worth at least 30'000 USD. I don't have any bills for that. I plan to use these devices there for 10 days and then take them back home. Will I get in trouble at the border in India on my way there or on my way back?
There are two answers here.
The correct thing to do would be to get an ATA Carnet ("Temporary Admission"), which basically certifies that whatever you're importing is going to go back out again, so you don't have to pay taxes and duties on it. India is a signatory, although there are some awkward conditions about them being "restricted to goods imported for display or use at approved exhibitions/ fairs and similar events", although this is apparently interpreted fairly loosely. Opting for this route will entail a full-on battle with Indian bureaucracy and you will almost certainly want to engage professional assistance. If you can't get a carnet, you'll need to post a Customs bond equivalent to the value of the taxes and duties you're not paying, which will be even worse to deal with.
In case that all sounds too scary, the practical thing to do would be to just import it with no declaration of any kind and hope Customs doesn't realize how valuable your item is. The odds of success are 99%, particularly if you're white (unfortunate but true) and can project the well-connected-businessman-so-don't-even-think-of-hassling-me vibe... but it could get seriously complicated and expensive if you luck out.
Information is available here. It appears that the limit for foreigners is Rs 35,000 (approximately US$600 as of 25 May 2014) plus 1 extra laptop on top of that.
There are notes that goods for commercial use are restricted and--satellite phones are strictly prohibited without a license.
If your devices include any type of high-end GPS or surveying equipment (not the GPS devices found in phones or computers), I'd watch out. Geographic data and maps are strictly controlled in India.