MTNL 3G is a only telecom provider which is providing limited 3g service in Delhi, India right now. Recently I got nexus one via a friend who was coming from US, and after some efforts I managed to get a Trumph 3G Jadoo connection and manage to configure 3G running on it, you can read my post on mtnl 3g manual settings configuration on nexus one.
Now after 3g setup on my nexus one, I started searching for 3g data coupons but it seems that in delhi no one buys 3g data coupons after buying the trumph 3g prepaid connection as the registration is has the lowest cost that is only 100 INR. No recharge coupons provider in delhi has the data card coupons for mtnl 3g connection, they all told me to go to the nearest sanchar hut for these data coupons.
After some hunting I managed to find one recharge coupons provider which had the list of recharge coupons values in which data card recharge values were also listed, I took a picture of that recharge coupons list, this may help you to get the data coupon recharge done by doing the recharge equal to the value listed under the data coupons value as shown in the image below.
I told the recharge coupons provider to recharge my card with a value of 250 which is the data coupon listed at 26th position in the image above. Once he did the 250 value recharge on my phone I got the message that I now have 500 MB of data usage as promised.