I evaluated the Nokia Lumia 900 over the past week and a half (I just returned the eval unit yesterday) and never experienced the connectivity issue that some users are experiencing. As stated on the Nokia Conversations site:
However, soon after this flagship device began selling, we identified a software issue. In short, a memory management issue was discovered that could, in some cases, lead to loss of data connectivity. This issue is purely in the phone software, and is not related to either phone hardware or the network itself. As a proactive and prudent measure, we decided to take immediate action. We have identified the issue, and have developed a solution.
They plan to roll out a software update on or around 16 April, that is this coming Monday, that will fix the issue. If you don't want to wait, then you can swap your device for one that has already been updated.
In addition to the software fix/replacement, Nokia is going to post a $100 credit to AT&T bills for every Lumia 900 buyer who purchased one already or who purchases one between now and 21 April. WOW! Nokia is essentially giving away the Nokia Lumia 900 to everyone who bought one already or who buys one by 21 April!
I'm not sure what will happen to those customers who paid full price for the Lumia 900 to buy it off contract or were not eligible for a contract.