We just found out that our server team built some servers with 2008 SP2 and some with 2008 R2. Is this going to cause problems?
AFAIK this is supported by Microsoft and I have seen multiple environments with a mixed OS setup. For example, during the transition to 64-bit with SharePoint 2007, it was fairly common for customers to split the upgrade into tiers, often starting with the SQL tier.
As long as the individual servers meet the minimum requirements I can't see it being a big issue.
If you are deploying customisations you may see inconsistencies between servers due to differences in the underlying OS, and from a configuration standpoint you may find that security changes etc. lead to additional administration overhead.
If Microsoft don't support a mixed OS setup that otherwise meets the minimum requirements I would be surprised and interested to know why.
This is a bit tricky since there is a difference between "causing problems" and "is supported by Microsoft".
First: It is not recommended to mix different OS's in one farm. If you can change it with reasonable effort, I would do it.
Second: If it is not possible to change the OS, at least make sure that every server role (like WFE, SQL, APP) has the same OS. It is not supported to mix OS within a role.
Third: If you have a problem with your farm and ask MS support to help, they will kindly please you to re-produce that issue on a farm with servers that have the same OS.
BTW: Troubleshooting errors caused by mixed OS is -let's say- ...interesting!