I am using Infopath with code-behind in Sharepoint Online (Office 365) since June 2012 (now is 2013, since January).
Commenting another answer with reference to:
- "Using code with InfoPath Services in SharePoint Online will not be supported by Office 365 moderators and this type of implementation may not be possible if it is required it be published to the Central Admin site. The article below will detail how to configure InfoPath Forums Services in SharePoint Online. Let us know if you have further questions"
Once more, it is written under condition of:
- "if it is required it be published to the Central Admin site"
This tells no more than about misconfiguration, misuse and unsupported usage of Sharepoint Online (Office 365). That is, in Sharepoint Online it is impossible to access Central Administration, and respectively create or deploy any solutions requiring deployment or configuration through Central Administration page (aka Administrator Approved Infoapth form templates or features unsupported by sandboxed apps, etc.)
Basically, I never had any problems with code behind. One should just follow some obvious restrictions which are basically avoiding to access something outide of Sharepoint Online site, i.e. to stick with using built-in features (web services) and with those deployed to SPO site.
Though, this is a theory but in practice there is an amount of maverick bugs especially after Sharepoint Online migration from 2010 to 2013:
There is also a webpage with officially shared (II mean by that that there are not shared and/or "approved" by Microsoft) bugs being processed by Microsoft team: