Nikon doesn't update firmware very often. No manufacturer does. Canon just tends to disable features on their lower end cameras so that's why "hackers" can later "re-enable" them through software. But officially canon has only recently started releasing firmware to add features such as variable frames per second.
The time limit isn't just Nikon. Canon's are limited too. Depending on who you believe the reason DSLRs have time limits is because if they didn't then they would be considered a "camcorder" and under European laws they would be taxed differently. Or the sensors over heat (not the camera body put the sensor inside) which could lead to failures. So the time limits help keep the sensor from overloading.
For me the limits aren't that big of a deal as I wouldn't want to stare at something for very long before cutting to a different angle or scene anyway.
But Niko you make it sound like it's just a canon thing... the CHDK hack for the rebel xti allows you to use the following ISO values
100 - 100 / 125 / 160 / 80
200 - 200 / 250 / 320
400 - 400 / 500 / 640
800 - 800 / 1000 / 1250
1600 - 1600 / 2000 / 2500 / 3200
as well as spot metering. This to me is a major improvement.
Yeah personally I don't know what to believe about the sensor overheating thing. That'd drive me crazy though, I wonder if the next batch will be any different. clillja's advice sounds like a good idea, would that work for you?