According to the PMBOK the primary output of the Planning Process Group is a Project Management Plan.  However, there are multiple processes that can be used to support the Project Management Plan.  Here they are in order from the 3rd Edition PMBOK:

3.2.2 PMBOK Section Name Relevant Agile practice, process or activity
.1 Develop Project Management Plan Combination of Product Backlog, Release scope, use cases, etc.
.2 Scope Planning We plan scope be defining the features to be included in a release.
.3 Scope Definition Scope is defined as the Product Backlog Items(PBI) that are included in the release.
.4 Create WBS The Work Breakdown is done as part of the PBI to Sprint Backlog Item(SBI) decomposition.  Again, we are aiming for SBIs that are no larger than 16 hours.
.5 Activity Definition Defined in the Product and Sprint Backlogs by User Stories and Use Cases.
.6 Activity Sequencing Activity is sequenced in terms of greatest business value coupled with greatest risk first.  Priority should be set by dollar value as discussed here.
.7 Activity Resource Estimation In Agile we can add or remove resources to increase or decrease velocity.  Depending upon the desired starting velocity (first few release) we can choose the team size based on estimates.
.8 Activity Duration Estimation Discussed in the posts here and here.
.9 Schedule Development Discussed in this post.
.10 Cost Estimation In an Agile environment we plan to deliver continuous value by way of release and iterations. As such, there is typically continuous cost.  Based on the resource estimation we do in .7 and the purchases from .20 we can estimate cost. This has been a sub-topic of a number of my posts.
.11 Cost Budgeting This method not used heavily in Agile.  Rely on Cost Estimation Schedule, CVE and CVP.   While Throughput Accounting is preferred over Cost Accounting PV, EV and AC can be calculated based on current release.  See PMBOK Chapter 7.2 for more information.  More on this in future posts.
.12 Quality Planning Involve QA upfront.  Need functional test for each PBI.  Build release to QA plan.  Quality sign off mechanism.
.13 Human Resource Planning Roles and responsibilities per Agile technique. See PMBOK Chapter 9.1 for more information.
.14 Communications Planning Should include a plan to update stakeholders with burndowns, impediments, roadmaps, etc. See PMBOK Chapter 10.1 for more information.
.15 Risk Management Planning Since risk is reduced in Agile, the planning process is highly simplified.  I will discuss more in future posts.
.16 Risk Identification I will discuss in future posts.
.17 Qualitative Risk Analysis I will discuss in future posts.
.18 Quantitative Risk Analysis I will discuss in future posts.
.19 Risk Response Planning I will discuss in future posts.
.20 Plan Purchases and Acquisitions Plan for hardware, third-party software, tools, etc.
.21 Plan Contracting Obviously required if necessary.

Now we can move on to the project execution part of this series…