While a combined view may have its uses (as Lucas notes in point 2 of his answer), I think most of the time you would want to see calls and messages separately because generally you'd be consulting them for different reasons:
Call-logs can only show the "who" and "when", not the "what". You might check them for missed calls, to add/update entries to your contacts, or to confirm that a particular conversation (or, at least, connection) took place. In all these cases, I can only see messages getting in the way.
Message-history will show the "what" (in addition to the "who" and "when"). While you may still check messages to update contacts and to "confirm an interaction", more often than not it will be to review what was actually said. Again, I can only see call-logs getting in the way of this process.
Another potential reason for keeping call-logs separate from messages is that -- traditionally -- call-logs usually list calls to/from everyone (perhaps with options to filter incoming/outgoing/missed), whereas messages are usually listed for a specific contact.
Having said that, it may be that "breaking the mould" by listing all calls and all messages together may offer new opportunities that the existing practices don't (e.g. seeing messages across all contacts listed in sequence). However, if doing this, I'd make sure there are easy ways to filter the list to get the "traditional" views that users are (currently) used to using.
If going with a unified list, I think the two obvious "high-level" filters would be:
Combined/Calls/Messages -- show all entries, or just calls or just messages.
All contacts/Selected user -- mix entries to/from everyone, or restrict to a specific user (as messages normally are shown). Would probably be useful to have a "Show [entries for] this user only" option either attached to each entry, or working on a selected entry so you don't have to scroll through all contacts to filter to just one person.
You might also consider filtering by:
- Incoming/Outgoing -- would apply to calls and messages. For messages this could be useful to find what you said to someone without what they said to you getting in the way (or vice versa).
Other selections that might prove useful (these might all appear in one filter/selector as "special cases"):
Missed calls -- incoming calls you didn't have a chance to answer.
Rejected calls -- incoming calls you chose not to answer.
Unanswered calls -- outgoing calls that weren't answered. I've not noticed this status being recorded on the couple of phones I've owned, but could be useful if the information is available.
Unreplied messages -- incoming messages to which you have not sent a reply (i.e. to find "dangling" conversations).
Unknown contacts -- calls or messages from/to someone not in your contacts database.
