There are a lot of differences between Sql Server 2000 and 2008. But as long as you don't use datatypes/functions introduced in 2005/2008 you should be fine. And of course you can't use SSIS/DTS and a few other new things introduced in newer versions. And it could depend on how you deploy your database. If it's scripts (t-sql) only (alter table, create table etc) there should be no problems.
Read more in these two MSDN Aarticles:
What's New in SQL Server 2005
What's New in SQL Server 2008