It has now been nearly two years since I obtained my greencard. If when you got your greencard you were married less than two years it means that the Green Card that you receive is a conditional greencard. The two most obvious sign that you have a conditional green card is of course the expiration date but also I believe it is marked as 'CF1' on the card.
It is hard to believe it has been that long already. To remove the conditions and to receive a regular 10 year card you must apply to have the conditions removed. This can be done by mailing the I-751 form (along with a cheque) to the service centre serving your state. You do not wait until the expiration date before applying. You can send the form in three months before hand. This will ensure that there is plenty of time to process the form and allows time to remmdy any problems that occur (which happenend to us).
We originally sent out form in November 2005. It was returned to us in December because I made a mistake on the check. Having sent it back we got a confirmation letter telling us that is taking about 6 months to process our form and that my card is automatically extended for a year. As it turned out I received the new card sometime in April.
In our case we had planned a trip to Mexico a month after the card had expired. This was Ok as we had already received a letter saying that the card had been extended. Anyway we had no problems coming through immigration in Houston TX with the expired card and the letter. They did however ask me how long it would until I received my new card. I promptly told them that I would get it when they decided to process my form (I could not resist).