Hi MrWilde,
If you're using a SharePoint list form, you don't have the Time only field type option. So, you should choose DateTime type. I'm guessing you did but that you are just using the Time component. This should still not cause a problem since it will insert today's date as the default. I just tried it and it promotes to SharePoint list columns just fine. If you create the columns from SharePoint as "Date Time" InfoPath only sees them as "Date" and not "Date Time" and you won't be able to change the type from InfoPath designer. I guess that's a limitation in the list forms.
In general, for most business process forms, SharePoint list forms are too limited. They don't support many of the controls (including repeating), have no support for extensions, and data can't be easily migrated - to list just a few reasons. But, the biggest issue is that they are limited by SharePoint list limits, so you can't extend them past ~80 columns (+/- 10 because it depends on the data types). They will just refuse to publish.
I prefer using a full-fledged InfoPath form and map to the list using CAML or qRules - the latter is more flexible, less time consuming, and supports roundtrips and mapping to multiple lists. Maybe you are already one of these techniques. If so, that's great. You can use a pure Time field type and map it as a string to your list. That's how the out-of-box sample timesheets worked in InfoPath 2003.
Send email to Support if you have qRules issues mapping time.
Good luck!