In the "Upgrading to BizTalk Server 2009 from BizTalk Server 2006" document there is a really useful table that explains exactly which edition of BizTalk 2006 (R1/R2) can be upgraded to which edition of BizTalk 2009. Not all combinations are possible or supported.

Here's the table from page 11 (with some added highlighting to assist those in a hurry):

BizTalk Server 2006/R2

BizTalk Server 2009 Evaluation Edition

BizTalk Server 2009 Branch Edition

BizTalk Server 2009 Developer Edition

BizTalk Server 2009 Standard Edition

BizTalk Server 2009 Enterprise Edition

Evaluation

No

No

No

No

Yes

Partner

No

No

No

Yes

Yes

Developer

No

No

Yes

No

Yes

Standard

No

No

No

Yes

Yes

Enterprise

No

No

No

No

Yes

The file can be downloaded from BizTalk Server 2009 Installation and Upgrade Guides.