Three typical approaches that have been used for CAD-GIS integration.

These three approaches of CAD-GIS integration I gleaned from www.esri.com.

File translation:
The conversion of data from one file format to another. This could be using an intermediate file such as ArcInfo Workstation in it's DWG-DXF-Coverage translator.
Direct read:
The data is read and converted on the fly into memory without an intermediate format.
Sharing access to a database:
Access to data on the fly is done by embedding an application program interface (API) on one system.

Applications that use both CAD and GIS.

An article by Petervan Oosterom on www.directionsmag.com contained information about CAD being used with GIS. I listed some applications I gleaned from that article.

Plan development:
Initial planning and layout of large infrastructures is often done in GIS. CAD is used to help with the engineering and construction. During the design phase, geographic information is often transferred from a GIS to a CAD system. After the design is done in CAD, it gets re-imported into GIS.
Visualization:
Adding CAD to GIS can add a more realistic, multidimensional view. Perhaps even adding a CAD model, such as a proposed building, to a GIS layout.
Data collection:
CAD can add detail and precision to remote sensing and photogrammetry in pertaining to objects such as buildings, especially sub-surface constructions which cannot be surveyed from the outside.

Common database management systems for GIS users.

I found on the gisjobs.com website in it's salary survey section, percentages of GIS users that took the survey that use different database management systems.

In North Carolina the top three were:
1. Access 63%
2. dBase 28%
3. MS SQL 27%

In North America the top four were:
1. Access 60%
2. (three way tie) dBase 24%, MS SQL 24%, Oracle 24%