I have a old gmail account, of email@example.com, which I have a significant amount of mail going into. I also have a new Google Apps account, firstname.lastname@example.org , which is obviously preferable. Is there anyway to get both of these into a single gmail inbox?
Set up forwarding the old account to the new.
When I changed domains I did this and went one better - I added a filter to label all mail that was forwarded from the old account. So now I can see it at a glance and go and change my details on whatever website is sending to the old address today.
I'd recommend you set up your Google Apps account to read your Gmail account via POP3.
You'll need to enable POP3 on your Gmail account. Then, configure your Google Apps account to read mail from the Gmail account using these settings. If you want to send mail from your old account as well, you should follow these instructions. I would recommend you configure the SMTP so that you don't see the "on behalf of" message.
You can read about the benefits of using POP3 over forwarding here.
Additionally, you may want to increase the frequency your Gmail account is checked.
What I do for my clients that I migrate to Google Apps-
[Google Apps Premier]
Use the mail migration tool to migrate all of their email appending a label to the new messages to note where they came from.
[Google Apps Standard]
Setup pop access on the old account. Go to the new account and add as an account to fetch mail from. Let it do the magic of moving everything over. Then proceed with the remaining steps.
Create a canned response to notify people what is happening "hey I have a new email address use this one! Don't worry I got what you sent anyways." Create a filter to trigger the canned resposne on emails from your old account. Also have it associate a new label so you can easily use it.
From old account - Setup mail forwarding, Push is much faster than the fetching.
When creating the filter on the new mail tell it to skip the inbox. In labs, enable "multiple Inboxes". In settings -> multiple inboxes remove the defaults and add "xyz@gmail" "label:whateveryourlabelyoumadewas". This will give you a visual "second inbox"